Our daughter has pretty smooth, unblemished skin. At least, that’s the way it appears when you look right at her. But when her husband takes pictures of her and puts them on line, it looks like she has freckles. I’m wondering what type of filter the camera has (UV?), and if that explains how her skin looks so much different like that. What’s the phenomenon at work here? I think I’ve seen advertisements for skin cancer treatments that look something like this, warning us that our skin is really more “damaged” than we think. I’m not sure what to make of this. What’s the straight dope? xo, C.
It’s probably that the freckles or blemishes are more visible in the infrared, which digital cameras are generally sensitive to, while our eyes are not. Most digital cameras have an IR filter to block out much of it, especially the longer wavelengths, but near IR still gets through. You can see this if you point a TV or VCR remote at your cam and press one of the buttons. You’ll see the IR LED light up bright as day in the cam’s viewfinder, but your eyes will see absolutely nothing looking at it directly. I’d wager the freckles would not be as visible in film photos, since ordinary color film is largely insensitive to IR.
Interesting. I visited a local drugstore where they had people taking photos with ‘UV damage revealing’ cameras. I was told that a couple things that looked like freckles in the photos were sun damage. And, as with your daughter, they’re not really visible to the naked eye. I wonder if it is just a matter of the type of filter that’s being used.
You can get the same visual ID of skin damage by standing near a “black light” and looking at your skin in the mirror. Be prepared for a surprise.
are these hidden freckles a real indicator of skin damage or just an artifact of skin pigmentation and different sensitivities to the spectrum?
Not completely accurate. We used to have terrible trouble with studio shots of synthetic fabrics that would fluoresce in infra-red, activated by the wide spectrum flashheads, and appear a completely different colour on film. Typically, greens would appear to be brown.
The only remedy was to filter the light source, it was too late once the dyes were activated by the IR, putting filters on the camera lenses had no effect whatsoever.
Are you sure it’s IR and not UV? If IR were causing the problem, I think it would be most noticeable under incandescent lights.
Ok - two hypotheses:** QED ** says it’s probably the IR, Carnac suggests UV. Do we have other suggestions or evidence to help sort it out?
A combination, UV rich source causing synthetic materials and dyes to fluoresce in IR. The problem was still there with incandescent (halogen) lighting and even daylight but not to such a great degree.
The primary difficulty was that you never knew which items in a collection would exhibit this phenomenon so a lot of stuff would have to be re shot to correct the problem.
It was the simplest expedient to take everything as suspect and use the same filtered lightsource for all clothing.
But you are correct in that it was fundamentally a UV source at the root of the problem.
Apologies but I failed miserably to say why I had brought up that problem in the first place.
It is of course that if skin pigmentation (melanin) has a tendency to display a similar effect to the above, UV rich light could cause it to reflect IR strongly but it would not be visible to the naked eye.
I can see problems in filtering it out without skewing the response spectrum.
Not at all sure how digital cameras filtering systems operate but they may be more narrow band and specific than optical systems.
Carnac is not one to quibble with Q.E.D.. I’m just offering my own (rather creepy) experience with a blacklight bulb.
Try it yourself. In a dark room, stand a few feet from a black light and look into a mirror. My facial skin appears blotchy all over. In normal lighting, nothing appears amiss whatsoever. (My niece got me to try this. Her skin under UV light appears perfect.)
A dermatologist told me this is an indication of UV skin damage, but the blotchiness surprises me.
Do I have this developing hypothesis correct? UV light tends to bring out features of the skin that are not normally visible under normal day light. Cameras that are sensitive to IR will pick up these features? If this were so, I’d certainly have to wonder why. UV is on the opposite end of the spectrum from IR. Why would something (skin patches) that fluoresces when irradiated with extra UV light, say, from a bulb, then radiate a large amount of IR for a camera to see? Is this the way skin/light works? Calling Dr. Dermatology! Dr. Dermatology, you’re wanted in the SDMB! Or, is this a job for Physicsman? Clearly, we mortals are struggling. Turn on the Bat Signal. (Just make sure there’s enough UV or IR in it so he can see it.)
It could be the UV from the flash (and xenon flash tubes do emit a lot of it) causing blemishes to fluoresce as described above, possibly in the IR or even in visible light. Pop into your local camera store and pick up a cheap UV filter (also called a skylight filter) and place it over the flash and try a picture.
But the problem wasn’t the film being sensitive to IR, but rather the film faithfully rendering the emission spectrum excited by the IR.
Tris
The UV in the flash causes the blemishes to give off IR? Is this analagous to the way IR comes into the atmosphere from the sun and when it hits the ground, it becoms much longer waves, effectiviely changing the type of wave it is, and trapping it inside the atmosphere in the bargain? One wave turning into another type of wave? Being modified, that is? Or are you suggesting that UV light can cause the skin to emit IR light? Here’s where I’m stuck.
That appears to be the case. Or at least skin pigmentation that may not be as evenly distributed as it appears to the naked eye is emitting IR caused by UV absorption
Definitely a dermatologists area of expertise.
That is a perfectly precise description.
The problem was that the colours recorded were not those “seen” and were the very devil to correct in print.
Possibly, yes. Or even visible light. You are correct in that this is analogous to shortwave IR causing materials to emit longer wave IR–it’s just another form of fluorescence. Remember that everything from radio waves to gamma rays are all forms of electromagnetic (EM) energy and that each EM photon has an energy level associated with it which is proportional to its wavelength. The constant of proportionality is Planck’s constant, h. When an atom or molecule of a substance absorbs a photon, its energy doesn’t just disappear, it goes into either heat or is stored in the electron orbitals; the latter is called excitation. Eventually, these excited electrons will drop back down to their normal energy level, and in doing so will emit a photon whose wavelength is the difference in the two energy levels times h. This is the fundamental mechanism behind all fluorescence. Since usually not all the exciting photon’s energy is stored in the absorbing electron, the emitted photon will normally have a longer wavelength. It can never have more energy, or a shorter wavelength.