Re: How do you measure the brightness of different colored lights?

Question: How do you measure the brightness of different colored lights? I'm trying to figure out which color of light is best seen in foggy conditions.

Response: Allison, thank you for your question. It really has two parts. What colour light do we see most easily ? and what colour light is least affected by fog ? Then we can see what colour light is best used in foggy conditions. I'll also touch briefly on how light is measured.

Let's take that last part first. Yes there are such things as 'light meters' and a whole lot of other light measuring instruments. They all need some sort of detector, something which gives a response when it absorbs light. The most common ones use metals which produce an electrical signal for example a voltage. This can easily be measured and related to the amount of light detected. One difficulty is that usually the response of the detector does depend on the colour of the light. This means that to be sure how much light is being measured we need to know how the detector behaves towards the different colours.

To make it simpler later on, it will be better if we used another expression for 'colour'. You are of course familiar with rainbows and how they show a number of colours in a sequence. The rainbow divides sunlight into these various colours because of an important feature of the light, namely its wavelength. We needn't go into this subject too deeply here. We can simply understand that one end of the spectrum of colours we call the rainbow has light of short wavelength, the blue end and the other longer wavelength, the red end.

When we measure visible light we often want to know how bright the light would seem to us, not necessarily how much of each colour or wavelength there is in the light. Just as with other detectors, our eyes' response depends on the wavelength. It is useful if the light meters we use respond to the different wavelengths in the same way as the eye and we can make sure they do if we need to. In this case the meter will imitate the eye and give an over-all brightness reading which predicts what we might sense if we looked at the light ourselves. Usually though, light meters are made to imitate the response of photographic film which is close to but not quite the same as the response of the eye.

However, you want to know a bit more, that is you want to know which particular colours are most easily seen by the eye. This web-site has some good diagrams of the eye and the way it responds to different colours or wavelengths. http://www.photo.net/photo/edscott/vis00010.htm One of the diagrams shows the over-all response that is the sum of the three separate colour detectors. It shows that the best response is in the yellow/green region of the spectrum that is around of 550 nanometers. This answers one of your main questions.

Now, what about seeing in fog. As you know, fog scatters light and makes it difficult to see a long way because of the interference of the scattered light. We need to know what wavelengths are least effected. Generally, if the particles, in this case droplets of water in air, are small compared with the wavelength of the light, then the scattering effect follows the behavior discovered long ago by the famous English gentleman-scientist Lord Rayleigh. He discovered that under these conditions, the shorter the wavelength of the light, the more scattering there is. This explains some familiar things about the world as we see it. For example it explains why the sky is blue and the sun gets redder as it sets. What is happening is that the blue light we see in the sky has been scattered out of the direct sunlight which is depleted of blue light and looks more red.

Does this happen with fog ? I think the answer is yes, but perhaps only to a small degree. It really depends on the sizes of the water droplets in fog . They may be a little bigger than the particles for which Rayleigh?s laws are useful and the way light behaves when it encounters fog droplets may be better explained by effects like refraction, that is, the bending of light rays entering and leaving the droplets. Anyway, you intend to use milk to imitate fog and this is a good idea. My word of warning though is that among other things the droplet sizes of milk are different from fog droplets and you will need to be careful about assuming fog will behave in the same way. One piece of advice is: use full fat milk. Shaken up to disperse the cream. This will mean that the bigger droplets of milk fat are being used and they will probably imitate fog a little better. That is, it more likely to look white whichever way you look at it dilute wheras low fat milk will look reddish if you look through it and blueish from the side just as in the case of the sky.

Because longer wavelengths are less likely to be scattered than short, the red end of the spectrum is best even if the effect with fog is small. However it is the combination of scattering and visibility that matters so the answer is probably somewhere in the yellow part of the spectrum.

Best of luck with the project

(note added by MadSci Admin: I happen to know a brilliant atmospheric physicist at Penn State, Dr. Craig Bohren, and I remember that he has studied this question. I found the following Web page that refers to Dr. Bohren's remarks: from Dr. Bohren - - - - John Link, MadSci Physics Admin)