ComedyPilot said:
Are any physicists on the forum ill?
Not feeling well, or just general sicklyness?
Well I am feeling a bit yucky. Hoping it will pass.
I did hope I'd started down the correct line of explanation earlier.
OK - First Part.
Brightness is a Human related number. There are two main ones - Photopic and Scotopic. One (Photopic) relates to daylight (ie bright light and depends on the colour receptors in the eye - the cones). The other (Scotopic) is the dim light condition when the rods are the dominant eye sensor.
So Brightness depends on the individual and whether rods or cones are the primary active sensors. In practice folks use an average over the human population for both Photopic and Scotopic eye responses.
Also Brightness depends on colour (because the eye responds differently to red, blue and green etc). So the colour make up of the source also has an effect.
Second Part
Brightness is human weighted light intensity per unit solid angle. Or HumanWatts/m2/Steradian. So with our assumption that the source is uniformly radiating over 4 Pi Steradians we can effectively just think about HumanWatts/m2 at the eye. Which for a given distance from the source and with the eye set to a predetermined aperture it's just watts.
Third Part
The eye is an imageing device. So the watts which appear at the retina depend on the area of the image on the retina, or at our standard distance - the area of the source. So if the humanwatts emitted by the source increase and the area of the emission stays the same then the source will look brighter. However the other requirement is that the colour make up of the source must remain the same for us to be able to conclude - more fuel = brighter. In the case of a hot element light bulb this isn't the case because the hot element acts as a black body and only the emissions that the eye percieves counts. So at low currents the light bulb only emitts infrared and it has brightness = zero, whereas as the current increases the colour starts off red and works it's way up to blueish before it blows. The brightness of the bulb then is a convolution of the drive power, the blackbody radiation curve and the human eye response curve. Radiant gas fires don't increase brightness with more fuel because the burn takes place over a correspondingly larger area usually. Lasers have huge brightness because they are close to being diffraction limited - ie they make the smallest of images on the retina and turning the power up doesn't change the area illuminated up much if at all. They also tend to be single wavelength - so no colour change.
So - have I got it correct? Questions?
the process of the emission of visible radation by the device results in a degredation of the emitter that is linearly inversely proportional to the intensity of the visible radiation emitted within the range of intensities investigated in the experiment and (c) the intensity of visible radiation emitted by the device remains constant over time until a critical point of degredation of the emitter is reached, at which point the emitter instantaneously ceases to be able to emit visible radiation.
