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When I looked into my projector when it was on the blue screen it left a red spot in my vision. I should not have tried it but all the colors left a red spot. Why not a blue or yellow spot was left?
You are describing the phenomenon of negative afterimages, a type of physiological afterimage, which is a result of how the cells in the retina work. Specifically, it is a result of the adaptation of photoreceptors.
According to the opponent process theory of color vision, our perception of color is controlled by two opposing systems: a magenta-green system and a blue-yellow system. For example, the color red serves as an antagonistic to the color green so that when you stare too long at a magenta image you will then see a green afterimage. The magenta color fatigues the magenta photoreceptors so that they produce a weaker signal. Since magenta's opposing color is green, we then interpret the afterimage as green.
There is a well-known illusion, called the negative photo illusion, which exploits exactly that. It is required not to move the eyes, because you need to bleach the photoreceptors in an exact place on your retina. This is why you have to fixate on a dot in the middle of a picture, otherwise the bleaching will not occur.
EDIT: I mention a two-axis (magenta-green system, blue-yellow) system, but the exact color system - which would predict which afterimage you would see - is most likely a trichromatic system, which includes red, green and blue channels, based on the principle of the three types of retinal cone cells. It is important to remember that this is a phenomenon in perception, and thus may, in some cases, not be reducible to the fact that there are three types of cone cells, or that bleaching of a photoreceptor has occurred. Please see the comment thread below for some clarification.
Read on to get answers and understand why you have this problem or why your child has it.
To begin, dermatitis refers to a group of skin conditions. If you had no idea of facial eczema then believe it is quite common.
Atopic dermatitis is one of the types of eczema whose symptoms are long-lasting and chronic. This type of dermatitis or eczema occur in various forms with an “inherited tendency to develop other allergic conditions, such as asthma and hay fever.” [niams.nih.gov].
The exact causes are unknown but Dermatologists believe that it is a combination of genetic or hereditary and environmental triggers including climatic or weather changes. From facts, this condition is characteristic of extreme itching. It is thus due to the itching that its victims end up with swelling, cracking and crusting characterized by a scaly skin.
Atopic dermatitis can affect anyone regardless of gender, sex or age but is more common during childhood or infancy.
Symptoms common to this type of eczema include the following but may vary from one individual to another.
- Itching (most common) and dry and scaly looking skin under eyes and even on eyelids,
- Scratching and rubbing as individuals try to relieve of itchiness
- Rashes on face elbows, behind the knees, hands and feet,
- Redness due to inflammation,
- Small raised bumps,
- Leaking fluid or ‘weeping skin’
Redness and dryness is also common in dermatitis round eyes, on eyelids, eyelashes and under eyes because of the same scratching and rubbing.
Contact dermatitis around eyes
Unlike atopic dermatitis, contact eczema occurs when the skin is directly exposed to a skin irritant or allergy triggering agent. In fact this type of eczema is associated with any substance that produces a reaction when it comes into contact with skin. Common examples of skin irritants include emollients, bad facial or eye beauty products like eyeliners, mascara, etc.
In reaction to these substances some individuals develop a skin rash around eyes or wherever the allergy triggered reaction occurs.
Symptoms of contact dermatitis or eczema include red patches, burning around eyes, dryness, crusting and sometimes blistering skin.
This is a bacterial infectious disease caused by Borrelia burgdorferifound in and transmitted by some species of blacklegged ticks.
Lyme disease is characteristic of erythema migrans – a rash that forms a red bulls-eye pattern on areas of the tick bites. In some individuals the rash can spread to different areas of the body apart from the affected areas. The rash sometimes clears in the center leaving a red patchy ring. Refer to the picture beside.
Other symptoms experienced are fever, headaches, chills and fatigue but these should not be concluded to be flu disease. According to Mayo Clinic if this disease is not treated after biting, the rash will spread and lead to serious complication, for instance, arthritis, joint problems and neurological diseases such as meningitis. During the later stages of infection, patients are also reported to get severe fatigue, hepatitis, irregular heat pulses, eye inflammation, nausea and vomiting among other signs/symptoms.
Reactions from certain medicines
Reactions resulting from medicines can be viewed as a form of allergic reaction to that specific drug or medication given via injections or taken orally. What happens in such medical situations is that, the immune system develops sensitization when it comes into contact with the allergy causing drug. Any further exposure to the drugs is what triggers the allergic reactions.
Normally, the reactions are side effects from use of such drugs (Epinephrine & Diphenhydramine) which results in skin developing a rash. People with a susceptible immune system sensitive to the named drugs are likely to develop a red bumpy skin rash. In some people it may be itchy and painful while others it may not. In some cases, people may get skin hives.
On the other hand, a rash will occur without allergic reactions. The rash may appear suddenly. It can also be painful and may lead to redness and peeling or make it look discolored. Dr. Wingfield E. Rehmus notes that the color could also be purple, blue or grey.
Vascular effects of alcohol abuse
How much alcohol do you drink in a day? Do you have any effects on your facial vascular (blood vessel) system? Abuse of alcohol has far reaching health effects besides the impacts it can have on one’s career and family personal life.
One of the effects of chronic alcohol abuse is telangiectasia characterized by a persistent red face (aka flushing) and prominent cutaneous blood vessels. According to Dr. Susan Simpkin of Dermnet New Zealand, telangiectasiaresults due to failure of the brain to regulate flow of blood vessels hence they become enlarged.
Why do I get red rash under eyes after drinking alcohol? This sign/symptom is attributed to Sulfur dioxide used in preservation of alcohol beverages. Some wines may also have histamines and tannins that are behind the reddening of face. Take a look at this photo taken from an adult with telangiectasia.
Facial redness is also common in polymorphism – people with a deficient Mitochondrial aldehyde dehydrogenase 2 (ALDH2) which plays a role in metabolism of ethanol.
Other health concerns or effects of too much alcohol include pruritus, hyperpigmentation symptomized by dark circles under eyes or round the eyes changes in nail colour, increased risks for skin cancer, liver cirrhosis among other many other conditions and syndromes.
NOTE: Keep in mind that alcohol can also heighten existing forms of allergies.
Atopic Keratoconjunctivitis (AKC)
Do you get red hardened and swollen redness under eyes especially the eyelid lining? This condition may be the cause. Generally, Atopic Keratoconjunctivitis is a genetic condition where your “immune system produces higher than normal antibodies in response to a given allergen,” [kellogg.umich.edu]
The condition can also lead to sensitivity to light, tearing and burning in eyes but does not mean that it is conjunctivitis. In severe scratching or itching, the eye may get damaged.
Allergic or contact conjunctivitis
Another possible cause is allergic conjunctivitis. Both forms of allergies (contact and seasonal allergens) can make the eyes inflamed or result in redness. Due to inflammation the eyelids (esp. the upper eyelids) may also become swollen and ignite a burning sensation. Other symptoms experienced include itchiness inside eyes/eyelids, red sore eyelids, gritty, etc.
Either one eye or both are affected and the symptoms may develop to severity.
Other causes and trigger factors of dry skin include the following:
- Hay fever
- Exposure to direct sun
- Injuries or trauma
- Excessive or chronic stress
- Some viral infections such as herpes
Itchy, Red Eyes? How to Tell If It’s Allergy or Infection
If you have red, itchy eyes and it’s hay fever time, you likely assume that allergies are causing the problem — and that you can treat it on your own with over-the-counter eye drops. Unfortunately, it isn’t as simple as that.
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Even if you find a guide to help you choose the best eye drops from the drugstore aisle, you may treat for allergies when the real problem is an eye infection.
Before you head to the drugstore, ophthalmologist Richard Gans, MD, shares what you need to know about these two very different conditions.
Allergies or infection? Here’s how you can tell
Whether ragweed or pet dander is the culprit, allergens affect the eyes in the same way.
Eye infections can come from many causes — virus, bacteria, parasite or fungus — and the symptoms vary with the cause, but in general, infections have a longer list of symptoms when compared to allergies.
The bottom line is that if anything more than tear-like fluids come from your eye or you feel eye pain, it’s likely more than allergies.
To get the right treatment, you’ll need your eye doctor to find out what’s behind your eye problem. Eye allergies aren’t contagious but they can be miserable to deal with. If it’s an infection, you run the risk of damaging your eye and/or spreading it to others.
How can you avoid eye problems?
You can minimize your risks for both eye allergies and infections. Keeping windows shut and other easily implemented strategies can help you survive seasonal allergies, while an air purifier can help you cope with indoor allergies. Conjunctivitis (pink eye) is the most common eye infection, caused by a virus or bacteria. Either way, it’s easily spread.
You can minimize your risks for both eye allergies and infections. Keeping windows shut and other easily implemented strategies can help you survive seasonal allergies, while an air purifier can help you cope with indoor allergies.
Conjunctivitis (pink eye) is the most common eye infection, caused by a virus or bacteria. Either way, it’s easily spread.
Frequent hand washing is just one way to help prevent pink eye and other contagious diseases. Taking care with makeup and contact lenses also helps prevent bacterial pink eye from spreading.
What to do when allergies or infections strike
Even if you fight the good fight, you may still sometimes need relief from itchy, watery eyes. Depending on your diagnosis, here are treatments that can help.
For allergies: Topical medications are usually better than general allergy remedies for treating eye allergies. Many allergy eye drops are extremely successful in treating symptoms. Some actually work to prevent symptoms by preventing the allergic reaction from getting started.
Your doctor may suggest short-term medications to help control inflammation, such as steroid or anti-inflammatory eye drops. Over-the-counter artificial tears also can help keep eyes moistened and flush out allergens.
For infections: Viral infections generally clear up on their own, but cold compresses and lubricating eye drops can minimize symptoms. Your doctor may prescribe antibiotic eye drops to treat a bacterial eye infection.
For eye infections caused by fungi and parasites, the medication will depend on what’s causing the problem. Your eye doctor can help sort that out.
Getting quick diagnosis and treatment is the key when you have irritated eyes. No matter what’s causing the problem, your eye doctor can help you find the right treatment and the relief you need.
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Equine Eye Problems: Types and Fixes
Equine eye problems can be caused by injury, allergies, viruses and parasites. Eye problems can be tricky because there are so many causes some mild and easy to treat, and some very serious. What makes it difficult is that the eye will show very similar symptoms for both mild and serious problems! Learn to tell the difference here.
Signs of Eye Problems in Horses
- Teary watery eyes(s)
- Gooey eye, green or yellow discharge
- Puffy, inflamed eye
- Red eye
- Horse rubs eye(s) frequently
- Cloudy eye
- Horse shows signs of pain or discomfort with eyes
- Sensitivity to light
- Dull eye
- Very crusty eye
Equine Eye Problems - Conjunctivitis
Conjunctivitis is inflammation of the tissues surrounding the eye and very commonly seen in horses. Equine conjunctivitis is often caused dust, flies and allergies, but can also be caused by a foreign body, an eye injury, herpes virus and the Leptospriosis bacteria.
The first thing that you will notice with conjunctivitis is a watery discharge. If the eye is tearingਊlot it will run down the horses face. The dried secretions will be blackish and crusty. If the horse is exposed to very dusty conditions or flies, using a good mesh type fly mask will help protect your horse’s eyes.
If you have an otherwise healthy horse and suspect dust to be the cause, this can often be cleared up by flushing the eye 2-4 times a day with an equine eye wash. If the eye(s) don’t clear within a couple of days or worsen or you suspect something more serious, get your horse to the vet.
It is common for a horse with a mild case of conjunctivitis to develop a secondary bacterial infection in the eye. Your vet will need to flush the tear duct. They do this by inserting a very small long tube into the lachrymal duct, just inside the horse’s nostril, and then flushing it out with a large syringe containing a medicated/antibiotic wash.
Your vet may give a mild sedative if your horse requires it. They will also examine the eye to rule out any other factors like a foreign body, an injury or any disease. Usually one tear duct flush treatment is often all it takes to clear up a case of conjunctivitis. Your vet may also give you a tube of eye ointment to apply to the horse’s eye if they think it’s needed.
Equine Eye Problems – Recurrent Uveitis
If the eye problem is caused by equine recurrent uveitis, or you suspect it may be, take your horse to the vet immediately. Recurrent uveitis is the leading cause of blindness in horses. Fast aggressive treatment can save your horse’s vision, if you catch it in time. To learn more about it, seequine eye problems - recurrent uveitis for details.
Equine Eye Problems – Injury & Allergies & Herpes Virus
It’s a no brain’er when your horse has an obvious eye injury to take them to the vet, but what about something more subtle like a scratch? The first thing you may notice is a teary eye, or that your horse does not want to open the eye all the way. Without an eye examination it is difficult to tell if the eye has a small scratch that is irritating the eye or they have been infected with the Leptospriosis bacteria, the cause of recurring uveitis.
Just like people, horses can get watery eyes from environmental and food allergies. A simple teary eye is less likely to be as serious as an eye that is showing other symptoms of irritation like inflammation or green or yellowish ‘eye goo’. If in doubt consult your vet.
The equine herpes virus does not cause eye problems, but rather a discharge from the eyes is one of the symptoms of the herpes virus (rhinopneumonitis). Vaccinating horses and pregnant mares is the best prevention. See horse vaccines for more details.
Equine Eye Problems – Eye Worms
Eye worms are a parasite called Thelazia spp. Thelazia is common in Europe and Asia and has only been recently seen in North America. This parasite is more often found in cattle but can spread to horses and humans. It is passed from one animal to another via flies, another good reason to use a fly mask for your horse and practice good fly control measures.
If eye worms are found the vet physically removes them manually with forceps and an anesthetic for the animal. The eye is flushed with an iodine solution and antibiotics.
Good to Know
I highly recommend that you keep a bottle of 'Clear Eyes' Sterile eye-care solution for horses and ponies in your horse first aid kit. It's very good for treating dust and allergies. See what else to add to your horse care kit here.
Currently there is a worming combination that has proven effective for cattle, but not for horses. There are no treatments approved for use on horses in the United States.
Most horses will likely suffer a mild conjunctivitis at one time or another during the course of their lives. Luckily it is easily treated with no ill effects, but because the symptoms of conjunctivitis are so similar to more serious eye problems, it is important that you as a horse owner be aware, so you can make the best judgment call at to when to seek veterinary treatment.
When you get out of the pool, why is your vision filmy and cloudy? Does this damage your eyes?
Ahh, the lazy days of summer. The sun is shining, the weather is warm and there’s nothing better than taking a dip in a swimming pool to cool down and relax. But, sometimes this quintessential summertime activity can leave your eyes itchy and irritated.
Your eyes are experiencing what the American Optometric Association calls chemical conjunctivitis—a swimming pool induced case of pink eye, only this version is just an irritation and resolves on its own without treatment. Chlorine can be blamed for causing the discomfort.
Chlorine, a basic ingredient for many household cleaning products, is contained in swimming pool chemicals, such as calcium or sodium hypochlorite, which act as ‘antimicrobial pesticides’, killing bacteria and controlling algae in the water.
Since pool water is open to contamination from the air and all the swimmers in it, chlorine, along with other chemicals such as iodine and algaecide, are added in high concentrations to fight off disease spreading organisms.
The average, backyard in-ground pool holds about 33,000 gallons of water, according to the Infinity Swimming Pool company. To ensure the water is clean, about 12- to 95-percent chlorine must be used because factors such as evaporation and sun exposure can cause the chemicals to degrade. According to the EPA, the average household cleaner contains about two percent chlorine, while standard bleach contains only about five percent.
The chlorine’s pH level is one cause of eye irritation. PH—a measurement of how acidic or basic a substance is—can be measured on a scale of one to 14. Since the pH of normal water is seven, this is considered a neutral pH anything less than seven is acidic and anything higher than 7 is basic. The pool’s pH must be at an appropriate level in order for the chlorine to be effective. A pH level between 7.2-7.8 is considered acceptable.
Chlorine is 100-percent effective against bacteria at a pH of 6.5, but this is far too low for our skin to handle—it would be like taking a refreshing dip in acid. At the level used in pools, chlorine is less able to break down bacteria, fats, oils and everything else we take into the water with us. According to the Australian Broadcasting Corporation’s Science Forum, if these compounds are not completely broken down, they can also cause stinging eyes and irritation.
Likewise, an over-chlorinated pool can put you at risk for eye irritations: If there is too much of the compound, our eyes can itch and sting from over-exposure. Normal chlorinated pool water should not do any permanent damage to eyes, but some may be more sensitive than others.
Although not usually a problem, there are all kinds of bugs living in swimming pools…and also in freshwater swimming sites, like lakes and rivers. One of these creatures, called acanthamoeba, can cause sever pain and damage to your eyes, and may even result in the need for a corneal transplant, says Dr. Burt Dubow from the website All About Vision.
Blurry vision as a result of freshwater swimming is usually attributed to the cleanliness of the water. Certain types of bacteria and algae can be especially harsh on your eyes. One of the most common is called cyanobacteria, also known as pond scum. Cyanobacteria is often a result of warm, stagnant waters—so the best way to avoid this is to swim in lakes that are sparkling clear and pristine.
Prevent irritation when swimming in fresh or chlorinated water by wearing goggles when swimming—especially if you wear soft contact lenses. Dubow says that water can change the way your contact lenses fit, making them too tight and causing eye problems.
Flushing your eyes with cool, fresh water after your swim is an easy way to calm any eye irritation. Saline eye drops are also a simple fix for sore eyes, making them an essential beach item for this summer season.
Watery or teary eyes and coronavirus
The same small study mentioned above found that some coronavirus patients experience overly watery eyes (epiphora), but wasn&apost able to definitively conclude they are a symptom of coronavirus.
If you have epiphora, tears may overflow from your eyes and run down your cheeks. Watery eyes can be caused by a variety of conditions, including bacterial keratitis, corneal ulcers, eye infections, glaucoma and macular degeneration. Excessive tears also are a common symptom of allergies.
Antihistamines, cool compresses, changing contact lenses and taking a break from staring at screens may help resolve watery eyes, but see an eye doctor if the problem persists.
Should I worry about a telangiectasia?
Telangiectasia is a condition in which broken or widened small blood vessels that sit near the surface of the skin or mucous membranes create visible patterns of lines.
For most people, these patterns, or telangiectases, neither cause any damage to overall health and nor require treatment. The widened vessels becomes weaker, causing bleeding and the eventual appearance of telangiectases.
However, they can sometimes signify a more severe medical condition and may warrant closer inspection.
In this article, we look at the causes, symptoms, diagnosis, and treatment of this condition, as well as the outlook.
Share on Pinterest Telangiectases on the legs are also known as spider veins, due to their appearance.
Unless a more serious medical condition is causing telangiectases to occur, they should not usually be cause for concern.
Telangiectases often occur in fair-skinned individuals with long-term sun damage. They can occur anywhere on the body but are most noticeable on the skin, some visible mucous membranes, and the whites of the eyes.
In most cases, telangiectases do not cause any symptoms. However, they can sometimes bleed. If this bleeding occurs in or near the brain, it can have severe effects.
The exact cause of telangiectases is often unclear, but several factors may contribute to their development.
- sun and wind exposure
- medications that widen blood vessels
- excessive alcohol consumption
- trauma to the skin
- surgical incisions
- prolonged use of oral or topical corticosteroids
The risk of a woman getting telangiectasia increases during pregnancy.
As her body grows to accommodate the developing fetus, it places significant pressure on the blood vessels.
Hormonal therapy and hormone changes resulting from the menopause or taking birth control pills can also lead to the formation of facial telangiectases.
Older individuals are more likely to have telangiectasia too, as the blood vessels start to weaken with age.
Share on Pinterest Liver disease can cause telangiectases.
Telangiectasia is sometimes a symptom of a more severe medical condition, such as:
- ataxia telangiectasia (AT), an inherited childhood disease that attacks the brain and other parts of the body
- Bloom syndrome, a rare genetic disorder that causes various symptoms, including telangiectases
- Osler-Weber-Rendu syndrome, or hereditary hemorrhagic telangiectasia (HHT), a genetic condition that leads to the formation of abnormal blood vessels in the skin
- port-wine stain, which is a large patch of discolored skin that is present at birth
- Klippel-Trenaunay-Weber (KTW) syndrome, a combination of port-wine stain, varicose veins, and enlarged soft-tissue cells , which is a chronic skin condition that causes redness and swelling in the face
- spider angioma, an abnormal collection of blood vessels near the surface of the skin
- Sturge-Weber syndrome (SWS), a rare disorder that can cause problems with the nervous system
- xeroderma pigmentosum (XP), a rare medical condition in which the skin and eyes become very sensitive to ultraviolet (UV) light
- liver disease
Connective tissue diseases
Connective tissue diseases can cause telangiectasia to develop on the face and the part of the hands and feet where the skin meets the nail.
Examples of connective tissue diseases include:
- Scleroderma: Limited scleroderma primarily blemishes the skin on the face, hands, and feet, while diffuse scleroderma also reaches the internal organs. Telangiectasia occurs more frequently in patients with limited scleroderma.
- Dermatomyositis: Telangiectasia can occur in areas of skin that get exposure to the sun, often including the neck, chest, shoulders, arms, and upper back.
- Lupus: People with lupus may get telangiectases in the nail folds and at the edges of discoid lupus lesions.
When to see a doctor
Visit a doctor when a telangiectasia points to a more serious condition.
If many small lesions appear on the face, it could be a sign of HHT, which is potentially a serious condition .
Limited patches over the breasts and buttocks can indicate poikilodermatous mycoses fungoides, a cancerous condition of T-cells in the immune system.
Telangiectases are fine, threadlike lines that are typically pink or red but whiten under pressure. Once telangiectasia appears on the skin, these tiny lines can range in color from red to blue or purple.
The distorted blood vessels generally measure between 1 and 3 millimeters (mm) in width. They are usually harmless but can cause itching and pain.
They commonly occur on the face, nose, chin, and cheeks, where they may cause facial redness.
Telangiectases are also often present on the legs, chest, back, arms, and legs. People often refer to those that appear on the legs as spider veins. A spider telangiectasia is a telangiectasia that has a red, central feeding vessel with outward branches.
Telangiectasia is common in people with no health problems and is generally a result of sun damage or aging.
Anyone experiencing severe cases of telangiectasia throughout the body with significant blood vessel enlargement should consult a doctor.
People should also seek treatment as soon as possible if they become aware of a family history of telangiectasia or have any bleeding or lesions in the mouth or eye area.
These symptoms could indicate a more serious condition.
To determine if there is an underlying medical condition, an individual may need some of the following tests:
There is no cure for telangiectasia, but the condition is treatable. Doctors will often devise a treatment plan based on the results of diagnostic tests.
For example, if acne or rosacea is the underlying cause, the doctor may prescribe an oral or topical antibiotic.
Many people choose to remove telangiectases, both for cosmetic reasons and because the condition can cause discomfort.
Doctors can use laser therapy, sclerotherapy, or excision surgery to remove telangiectases.
Laser therapy is minimally invasive and generally the most straightforward treatment for facial telangiectasia and broken capillaries. Laser ablation can seal the widened blood vessels. This procedure does not cause much pain and the recovery time is short.
Sclerotherapy is a more effective procedure for larger veins, so it is the standard treatment for telangiectases on the legs. It is a minimally invasive procedure in which a dermatologist injects salt water or a chemical solution into the affected veins. The veins will harden and disappear.
The treatment is effective but can take a long time. Insurance companies rarely cover the procedure, as they consider spider veins to be cosmetic in nature. The treatment can therefore also turn out expensive.
One injection can treat an inch of a vein. People may need between five and 50 injections, depending on the size of the area that the condition affects.
This low-cost treatment does not require a stay in the hospital, as there is no recovery time or anesthesia. It may also be a good option for facial spider veins that are larger than regular broken capillaries.
Excision surgery can remove widened blood vessels, but this procedure has a more extended recovery period and causes considerable pain.
Flash blindness is caused by bleaching (oversaturation) of the retinal pigment.  As the pigment returns to normal, so too does sight. In daylight the eye's pupil constricts, thus reducing the amount of light entering after a flash. At night, the dark-adapted pupil is wide open so flash blindness has a greater effect and lasts longer.
Depending on the source consulted, term may exclusively refer to a temporary condition or may describe a potentially permanent one. Some sources, such as NATO and the U.S. Department of Defense, state that "flash blindness" can be temporary or permanent.  Other sources restrict the use of the word to temporary, reversible vision loss: ". These are, in order of increasing brightness: dazzle, after image formation, flash blindness, and irreversible damage."  The United States Federal Aviation Administration in Order JO 7400.2 defines flash blindness as "generally, a temporary visual interference effect that persists after the source of the illumination has ceased." 
Because vision loss is sudden and takes time to recover, flash blindness can be hazardous. At some sporting events such as figure skating, fans are cautioned to not use flash photography so as to avoid distracting or disorienting the athletes. Also in aviation, there is concern about laser pointers and bright searchlights causing temporary flash blindness and other vision-distracting effects to pilots who are in critical phases of flight such as approach and landing.
The bright initial flash of a nuclear weapon is the first indication of a nuclear explosion, traveling faster than the blast wave or sound wave.  "A 1-megaton explosion can cause flash blindness at distances as great as 13 miles (21 km) on a clear day, or 53 miles (85 km) on a clear night. If the intensity is great enough, a permanent retinal burn (photic retinopathy) will result." 
It is unclear whether pain is directly associated with flash blindness. [ citation needed ] Reaction to flash blindness can be discomforting and disorienting. The retina has no pain receptors.  Nonetheless, psychological pain may very well be present. It can cause amplified stress levels but usually fades.
Welders can get a painful condition called arc eye. While it is caused by bright light similar to flash blindness, the welder's arc lasts for much longer than a flash and emits ultraviolet rays that can damage the cornea. Flash blindness, in contrast, is caused by a single, very brief exposure which oversaturates the retina, and is not usually accompanied by reports of pain.
While bumpy moles aren't always the cancerous ones, that's not to say you shouldn't point them out to your doctor. "Some skin cancers, such as basal cell carcinoma, do form raised bumps," Fenton says. "So don’t ever hesitate to point out anything that you feel is different or abnormal."
While sun exposed areas are at a greater risk for skin cancer, Fenton tells me some forms of cancer — such as melanomas — can form in areas that rarely see the sun. "That’s why during a skin check, I always look at the bottom of the feet, between the toes, and beneath the underwear." If you notice anything where the sun don't shine, such as a mole between your toes, have it looked at right away.
What to know about freckles
Freckles are small, harmless marks that appear on the skin. Genetics and sun exposure are the primary causes of freckles.
Some people are more likely to get freckles than others, depending on their genes and skin type. If a person is genetically more likely to develop freckles, exposure to sunlight can make them appear.
Freckles are common in children and may disappear or become less noticeable as they grow up.
In this article, we look at what causes freckles, how to distinguish them from other similar marks, ways to remove or lighten them, and when to see a doctor or a skin doctor called a dermatologist.
Freckles appear when melanin, the pigment that gives skin its color, builds up under the skin. Freckles may look brown, red, or tan.
Sun exposure and genetic factors make some people more likely to develop freckles:
Share on Pinterest Sun exposure and genetics can cause freckles.
A person’s skin cells produce extra melanin to protect the skin from sun damage. This is why freckles tend to appear after sun exposure.
Freckles can appear over a large area of skin and can reappear or become darker in the summer months. Freckles often fade or disappear in the winter months, when new skin cells replace old cells.
Freckles develop on areas often exposed to sunlight, such as the:
Genetics also play a leading role in who is more likely to develop freckles based on which type of melanin their body produces.
The body can produce two types of melanin called pheomelanin and eumelanin. Eumelanin protects the skin from UV rays, but pheomelanin does not.
The type of melanin the body produces depends on a gene called MC1R.
- People with dark hair, eyes, and skin usually produce mostly eumelanin and are less likely to develop freckles.
- People with red, blonde, or light brown hair and who have light-colored skin and eyes usually produce mainly pheomelanin and are more likely to develop freckles.
Freckles are not dangerous. However, as people with freckles have skin that is more sensitive to sunlight, they should take extra care to protect their skin from the sun.
Freckles can look very similar to other marks that develop on the skin. For example, they can look like sun spots, also known as age spots, or liver spots.
Sun exposure is a primary cause of both freckles and age spots. Age spots are typically larger than freckles, are more clearly defined, and tend to appear in older adults.
While freckles are more widespread on people with light-colored hair and skin, age spots develop on people with a wider range of complexions.
Moles are usually present from birth, but people can develop them throughout childhood and teenage years. Moles are generally darker than freckles and can be flat or raised off the skin.
This table shows the differences between common marks on the skin:
|Appearance||Flat, can appear in clusters over a large area||Flat, can appear in clusters||Flat or raised, can appear on their own or in groups|
|Cause||Genes and exposure to sun||Exposure to sun||When skin cells grow in clusters|
|At which ages do they appear?||Can first appear at 2–3 years old||More common in people aged 40 or older||From birth, or during childhood and adolescence|
|Where can they appear on the body?||Areas of sun-exposed skin, commonly the face, neck, chest, arms, back||Areas of sun-exposed skin, typically the face, hands, arms, shoulders, back||Anywhere on the body|
|Shape||Irregular shape with well-defined edges||Well-defined edges||Round or oval, well-defined edges|
|Color||Tan, brown, or red||Tan, brown, or black||Light to dark brown, or black|
|Size (diameter)||1–2 mm or larger||2 mm or larger||Usually less than 6 mm|
|Changes to expect||Can fade in winter and become darker in summer||Will stay the same may get darker if left unprotected in sunlight||Will stay the same can sometimes disappear over time|
People can prevent or reduce the appearance of freckles by protecting their skin from the sun.
Protecting the skin from sunlight will not reduce the appearance of existing freckles, but it can prevent new freckles from forming.
The American Academy of Dermatology (AAD) recommend the following tips to protect the skin in sunlight: