Chapter 4- Photometry Flashcards
Difference between radiometry and photometry
Radiometry is concerned with the power produced by electromagnetic radiation, not taking into account its effect on vision.
Photometry deals with the effect this radiation has on the visual system.
The basis for photometric measurements is the ____ curve, which shows that certain wavelengths are more efficient at stimulating the visual system than others
The Photopic luminosity curve
The basic unit of photometry is the ___, a measure of ____.
Lumen, a measure of luminous power.
How many lumens per watt at 555nm?
680 lumens per watt at 555nm
This is the peak of the photopic luminosity curve
Other wavelengths are less efficient, producing fewer lumens per watt.
The basic unit of radiometry is the ___, measured in ___
The basic unit of radiometry is the watt, measured in radiant power.
Luminance quantifies what
The amount of light coming off a surface, such as a piece of paper in a specified direction.
Common units of luminance
Candelas per square meter and foot lamberts
What type of surface shows the same luminance regardless of the angle at which the luminance is measured?
Cosine surfaces/lambert surfaces/matte/perfectly diffusing surfaces. From whichever angle you view a matte surface, the brightness is also the same.
The perceptual attribute most clearly associated with luminance is
brightness.
Define: Luminous power Luminous intensity Luminance Illuminance
Luminous power- Total light power produced by a source in all directions.
Luminous intensity- Light power produced in a solid angle by a point source.
Luminance- Luminous intensity per unit projected area of an extended source (not a point source)
Illuminance- Luminous power falling on a surface
Units each are measured in: Luminous power Luminous intensity Luminance Illuminance
Luminous power- lumens
Luminous intensity- candelas
Luminance- candelas per square meter (foot-lamberts)
Illuminance- lumens per square meter (lux) or lumens per square foot (foot candles)
True or false. Illuminance is not affected by the surface on which the light falls.
True
As a surface is moved away from a point source, the number of lumens falling on it _____
decreases with the square of the distance, resulting in a decrease in its illumination.
Illumination falling on a surface= Intensity of the point source/distance squared
E=I/d^2
How do luminescent/LED sources work
Filled with gases whose atoms are energized into an excited state by a voltage differential applied across the tube. The unstable gas atoms release energy when they spontaneously revert to their unexcited state.
This energy is in the form of short wavelength UV radiation, which is absorbed by a phosphor coating on the inside of the bulb. The phosphor then re-emits the energy as a longer wavelength, visible light.
Compare relative energies of incandescent and fluorescent light bulbs
LED/fluorescent sources have an uneven spectral distribution with energy spikes across the wavelength spectrum. This is why they may be unsuitable for administering color vision tests.
An incandescent source shows a relative concentration of energy at long wavelengths (500-700nm)
LED energy tends to be concentrated in ___ spectral region.
Blue spectral region. This accounts for the bluish light cast from devices such as computer screens, which use LED sources.
A blackbody radiator describes what
Describes sources of electromagnetic radiation, including light sources.
Blackbody radiator trend that occurs as you increase the temp
As you increase the temperature, the wavelength peak decreases, and the area under the spectral distribution curve, which represents power, increases.
Color temperature refers to
Refers to the temperature in Kelvin of the blackbody radiator, not the light source.
Most appropriate when specifying incandescent light sources.
Ex: Warm white, bright white, daylight.
When to use the term “color temperature” and when to use “correlated color temperature”
Color temp- when specifying incandescent light sources.
Correlated color temp- When specifying LEDs. The correlated color temp of a fluorescent light tube depends on the nature of the phosphor that coats the bulb.
When checking color vision, should you use illuminant A, B, or C?
C. Higher blue light content.
Which lens material provides protection against UVA and UVB?
Polycarbonate. It’s percent transmission is lower than CR39 and glass. More light will get through the CR39 and glass.
Role of neutral density filters
Appear grey.
Transmit all wavelengths equally, thereby minimizing color distortion. Objects seen through this filter would retain all of its color.
What does the transmission curve look like for a neutral density filter?
Relative Transmission, the Y axis.
Wavelength on the X axis.
Flat line. Bc it does not favor one wavelength/color.
Example of subtractive color mixtures
Overlapping two filters. Only light common to the two filters- the area of overlap- is transmitted.
Note that the two overlapped filters transmit less light than either filter by itself.
Example of additive light mixtures
The output of two separate light sources is combined on a screen to produce an additive light mixture.
