Photometry Flashcards
Power produced by a source of EM radiation
Radiometry
Deals with the effect that the radiation has on the visual system
Photometry
What is the difference between radiometry and photometry
Radiometry does not take into account the “visibility” of the source
What is radiometry important for
Surgery
Describes the relative sensitivity of the eye to different wavelengths in the visible spectrum
Photopic luminosity curve, V(lambda), CIE luminosity function
What is the fundamental difference between photometry and radiometry
V (lambda)
At what wavelength is the eye most sensitive to
550
What does the photopic luminosity curve tell us
Certain wavelengths are mroe efficient at stimulating the visual system than other wavelengths of light
What is to be said about a blue light source at 10W, and wavelength of 400nm, and a red light source at 10W and 600nm wavelength?
Radiometrically equal but photometrically different
How many lumens a certain light source produces per watt
Luminous efficiency
Equation for luminous efficiency
LE x (680 lumens/W) x W
Total amount of light that emerges in all directions from the source
Luminous power
What is the unit for luminous power
Lumens
Light power produced in a given direction by a point source
Luminous intensity
What is the unit for luminous intensity
Candela (cd)=1 lumen/steradian
Steradian is cone
The amount of light coming off of a surface in a specified direction
Luminance
Unit of luminance
Candeal/square meter (cd/m2)
-nits and apostilb
The luminous power that falls on a surface
Illuminance
Unit of illuminance
Lumens/square meter (on/m2)=lux
Total light power produced by a source
Luminous power
Radiometric equivalent to luminous power (lumens)
Radiant power (joules/second or Watts)
Light power produced in a solid angle by a point source
Luminous intensity
Radiometric equivalent of luminous intensity
Randiant intensity (watts/steradian)
Radiometric equivalent of luminance
Radiance (watts/steradian/square meter)
Radiometric equivalent of illuminanace
Irradiance (watts/square meter)
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
Inverse square law
E=I/d^2
E=illumination falling on the surface
I=intensity of the point source
D=distance from the point source to the surface
Criterion for luminous intensity with different surfaces
- how much light illuminates the surface
- distance from the light source
- reflectance of the surface
- tilt of the surface relative to your line of sight
Reflects all the light in one direction. In this case, the angle of reflection=the angle of incidence. These surfaces appear shiny like a mirror or polished chrome
Speculation reflection
Scatter reflected light in all directions. These surfaces appear matte or dull. Examples include a wall painted with matte paint or a non-glossy piece of paper
Diffuse reflector
Relationship between illuminance and luminance for a lambertian surface
If a 100% reflecting (pure white) lambertian surface is illuminated by 1.0 lux, it will produce a luminance of 1/pi nits or 1 apostilb.
One apostilb
1 aps= 1/pi candelas/m2=1/pi nits
When does apostilb apply
Only to lambertian surfaces
Asb=
1/ (cd/m2)
This quantity is the amount of light falling on the retina
Retinal illumination
What is the unit for retinal illumination
Roland’s (Td)
What is a troland (retinal illuminance) directly proportional to?
Object illuminance and pupil area (not diameter)
Formula for retinal illuminance
Td=(luminance of the object in nits)x (pupil AREA in mm2)
Why does retinal illuminance remain constant for any object distance?
While the illuminance, that is, the amount of light entering the eye decreases with the square of the distance, the area of the retinal image also decreases with the square of the distance. The retinal illuminance therefore remains constant
What what wavelengths do incandescent lights perform better
Larger
At what wavelengths do fluorescent lights perform better?
Erratic, not able to tell
At what what wavelengths do LED lights perform better?
Blue spectral region
A theoretical construct that is convenient for describing sources of electromagnetic radiation, including light sources
Blackbody radiator
As temperature of blackbody radiator increases
The peak wavelength decreases and the area under the spectral distribution curve, which represents power, increases
What color is at 2000K
Yellowish
What color is at 10,000K?
White/blue
As temperature increases on a blackbody radiator what happens to wavelength
Decreases
A blackbody radiator with a temperature of 2000K has most of its power concentrated at _____ wavelengths
Longer
A blackbody radiator with a temple rather of 10000K has its power concentrated at _____ wavelengths
Shorter
Warm white
2000K-3000K
Cool white
3100K-4500K
Daylight
4600K-6500K
Advantage to higher color temperature
We see better at night with 4000K LEDs than with 2700K high pressure sodium bulbs
Disadvantages to higher color temperature
3 times more scattering at 465nm than at 600nm. Rayleigh scattering, causing more area of glare
What can high color temperature cause
Discomfort glare (annoyance glare)
Who is most affected by annoyance glare from higher color temperatures
Presbyopia and cataract patients
Cone, or daylight sensitivity peaks at ___
555nm
Rods, or night time sensitivity peaks at ____
507nm
The peak luminosity of photopic vision is _____ at peak response of 555nm
680 lumens/W
The peak luminous efficacy of scotopic vision is _________ at a peak response of 507nm
1700 lumens/W
What is the eye more sensitive to at mesopic and scotopic conditions
Shorter wavelengths or higher color temp
May absorb/reflect some wavelengths that are incident upon it
Colored filters
A filter that passes only a certain specified spectral band of light, such as only pure green (550-555)
Bandpass filters
A filter that passes only a narrow spectral band of light
Narrowband filter
A filter that passes only a broader spectral band of light
Broadband filters
A filter that transmits long wavelengths of light, but not shorter wavelengths
Long-pass filter
A filter that produces a very narrow band of light based on the interference principle of light
Inference filters
Color filters subtract light. Two filter combos in sequence transmit less light than either one by itself
Subtractive color mixtures
Light from different filters summed. Light is projected through two different color filters and combined on a screen to produce an additive mixture. It has more light that either one by itself
Additive color mixtures
Transmits all wavelengths equally, and minimizing color distortion
Natural density filters
How are natural density filters specified
By their optical density
OD=log (1/T)
T=transmittance of the filter
Application of neutral density filters
used in combination with high pass filters that block UV radiation in sunglasses to minimize color distortion for patients who have anamalous color vision
