We can test these ideas in our field of vision test box! We have started to test the angular field of view (results later) but if instead of just using standard LEDs as the light source at different angles, we control the colour and brightness we can investigate how faint a light we can see and when we stop being able to see colours at different angles...
To do this we have two problems:
1 - How bright is a light? how do we measure brightness?
2 - How can we make a faint light and make lights of know brightness (or at known relative brightness)
First (we'll deal with the second issue in another post) what do we mean by brightness? Bright enough to see by? Bright enough to read by? An obvious (to a physicist!) definition would be a measure of how much light energy a light gave out, but that is complicated because different colour photons have different energies (ultra-violet photons have much more energy than red photons for example, but we cannot see in ultra-violet so energy by itself is not very useful for measuring brightness). We could count the photons themselves, which would be better, but would not help us define a bright light because again, we do not see all photons equally - the cone and rod cells have different ranges of sensitivity (and again do not respond to ultra-violet or infra-red photons...). What we want to measure is how many visible photons come from a given source or area and weight this by how sensitive our eye is to them.
In fact this makes it easier - we just need a standard light and compare all other lights to that!
Scientists wanted to do this a long time ago so they picked the most obvious standard light for the time - the Standard Candle and this remains (more or less) the official unit for luminous intensity (brightness) or amount of light emitted per unit solid angle.
There are some other units that are important when quantifying how bright something is...
The further away the candle is, the fainter it appears because the light is spread out - we are actually more interested in how much light is arriving in our eye than how much is given out by the light. We define the Candela (cd) in terms of a rather precise type of light source now, but a standard candle emits light with an intensity of about 1cd and this is a measure of the energy of visible light per unit solid angle - this is the luminous intensity of the light.
We measure the total amount of light given out by a light source, the luminous flux (measured in Lumens, lm), by the luminous intensity (measured in Candela, cd) multiplied by the solid angle over which it is emitted. Finally, we measure the amount of light arriving at a surface (measured in Lux, lx) in lumens per unit area (!) All of this has to be defined in terms of a frequency of visible light and related to the sensitivity of the eye! It's complicated!
So if you are comparing light bulbs to buy, you are interested in how many lumens they give out, if you are interested in whether you can see a faint light you are interested in how many lux it gives at your eye.
We tested this initially last week and used a iPhone Luxmeter app to measure the brightness of a candle (standardish!) in Lux and compare to one of our LEDs but I lost the results! Oops - we'll have to repeat it...
UPDATE!
Some of us remembered the LED and the candle both gave readings of about 1 lx at a distance of 1m (reassuringly the candle gives out 1 cd!)
As memory is not very infallible, I repeated it it in the darkest room at home and got 1-2lx for the candle and 1 lx for the LED - the candle gave 4lx ish at 0.5m (it should be 4 times more lux at half the distance) so it looks like our LEDs are similar to standard candles given the uncertainties associated with Luxmeter at low light conditions and the problems of background light and reflections....)
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