Lecture 1 - Photometry, Radiometry Flashcards
Is radiometry objective or subjective?
Objective
Power produced by EMR; Doesn’t take into account visibility
Radiometry
Is photometry subjective or objective?
Subjective
Effect that radiation has on visual system; Perception - light bright enough or not?
Photometry
What is the fundamental difference?
The multiplication of radiometry and photometry = V(wavelength)
What is the luminous efficiency?
How bright/efficient the light is
Does the wavelength of light matter when talking about the efficiency of it?
Yes. Some wavelengths of light are more efficient at stimulating the visual system than others
What does the photopic luminosity curve show?
Visible spectrum to different wavelengths
Example:
At 550 wavelength, what color is the human eye most sensitive to?
Green
______ can be equal, but ______ different
Radiometry (power)
Photometry (wavelength)
Example:
At 400 nm wavelength, P = 10W Blue light
At 600 nm wavelength, P = 10W Red light
What will you see?
You’ll see red, but not blue because blue = 0 lumens (or Watts) and red = 4216 lumens (or Watts)
Example:
400 nm @ 10W is equal to how many lumens?
(Luminous efficiency) x (max visual system = lum eff = 680 lumens/W) x (Power in watts) 0 x (680) x (10) = 0 lumens
Example:
600 nm @ 10W is equal to how many lumens?
(Luminous efficiency) x (max visual system = lum eff = 680 lumens/W) x (Power in watts)
0.62 x (680) x (10) = 4216 lumens
Photometry units of luminous power and direction of light
Lumen (lm)
All directions
Photometry units of luminous intensity and direction of light
Candela (cd) = 1 lumen/steradian
Given direction
Photometry units of luminance and direction of light
Candela/square meter (cd/m^2) = nit/apostilb (asb)
Coming off surface
Photometry units of illuminance and direction of light
Lux = lumens/square meter (lm/m^2)
Falls on surface
What unit is radiance power in?
Watts
As surface moves away, number of lumens falling on it decreases with square of distance and illumination
Inverse square law
Equation to find Inverse Square Law
E = I/(d^2)
Illumination falling on surface = (intensity of point source)/(distance from point source to surface^2)
What are the 4 things you need to know to figure out the amount of light that is, it’s brightness, the luminous intensity with different surfaces will depend on?
1) how much light ILLUMINATES surface
2) DISTANCE from light source
3) REFLECTANCE of surface
4) TILT of surface to line of sight (@ certain angle)
A source with which of the following color temperatures will appear most red?
2000K
The color temp of an incandescent bulb is 3300 degrees Kelvin. This means that:
The bulb’s spectral distribution matches that of a black body radiator whose temperature is 3300 K
A neutral density filter transmits 50% of 450 nm light that is incident upon it. If 40 W of 550 nm are incident on the filter, how many Watts are transmitted?
20
The least amount of distortion is present with sunglasses made with what kind of filter?
Neutral density filter
Which of the following light sources has more energy concentrated in the blue region of the spectrum?
LEDs
A patient whose retina contains only rods (i.e., a rod monochromat) views the following four patches of monochromatic light: 440, 510, 555, and 565 nm. Each patch emits 20 quantral absorptions. Which is brightest?
510 because it’s closest to 507 nm
Assume that the absorption of 10 quanta of 510 nm by an eye that contains only rods (i.e., the eye of a rod monochromat) results in vision. If the wavelength were 450 nm, how many quanta would need to be absorbed to produce vision?
10 quanta
Question could also be read a little differently, so the answer could also be greater than 10 quanta
Your patient, a rod monochromat, has asked for advice regarding tinted lenses. You should advise a darkly tinted lens of which color?
Red
(Rods are more sensitive at lower wavelengths; the photochromatic interval curve is the difference between photopic and scotopic. Where they merge, it’s at 0, and at 650 nm (red))
In the photchromatic interval curve, where is the difference between photopic and scotopic the least?
At 650nm (red)
Under scotopic conditions, a blue and yellow flower appear equally bright. Under photopic conditions, which flower will most likely appear brighter?
The yellow flower
Photopic = 555 peak sensitivity is around the green/yellow mark
A patient has degenerative disease that has destroyed her cones. The expected visual acuity for this patient is closest to:
20/200 (her peripheral vision is still in tact for the most part, but her central vision is pretty much gone)
When foveally fixated by a person with trichromatic vision, a small target (0.5 degrees) of which of the following wavelengths will be least visible?
430 nm
No blue cones in the fovea, up to 0.3-0.5 degrees eccentricity and no rods => have green and red cones
Under daylight conditions, a red and green surface looks equally bright. Compared to the green surface, the red surface most likely emits:
More energy (Daylight = photopic sensitivity curve => need more quanta to be equal b/c it peaks at 555, and you are around 650, so you have to add more quanta to get up to the peak again)
Photopic and scotopic thresholds are most similar to each other for which wavelength?
650 nm (Photochromatic interval b/w rods and cones at 650 nm = 0)
Which of the following classes of cones contributes least to the photopic spectral sensitive curve?
S-cones
Blue cones are in much less amounts than red and green cones
What are the 2 types of reflective properties of a surface?
Specular reflector
Diffuse reflector
What direction does light reflect in specular reflection?
In one direction (angle of reflection = angle of incidence)
What do specular reflector surfaces appear as?
Shine, like a mirror or polished chrome
What direction does light reflect in diffuse reflection?
In all directions
What do diffuse reflector surfaces appear as?
Matte or dull (not glossy)
Ex: wall painted with matte paint, or a non-glossy piece of paper
If a 100% reflecting (pure white) lambert Ian surface is illuminated by 1.0 lux, what luminance will it produce?
Luminance = 1/3.14 (pi) nits
Units of illuminance
Lux = lumens/m^2
1 lux = how many nits after it hits a 100% reflector?
1/pi nits = 0.318 nits
One apostilb is equal to how many nits?
1/pi nits = 0.318 nits
When luminance is measured in apostilbs, what’s the relationship between illuminance and luminance?
1 lux onto a 100% reflector surface gives off 1 apostilb (asb)
Equation for apostilb to nits
1 apostilb = 1/pi candelas/m^s (or 0.318 nits) = 1/pi nits
White surfaces reflect how much of the incident of light?
Nearly 100%
If a grey lambertian surface reflects 50% of the light, what would be the resulting luminance?
50%
When 1 lux of illluminance falls upon a grey lambertian surface that has 50% of reflectance, what is the luminance that comes off of the surface?
0.5 apostilb (1/2 reflectance)
The apostilb is the second unit for _____, and it applies only to _____ surfaces.
Luminance
Lambertian
Finish this equation:
Asb = ?
1/pi candela/m^2
Finish this equation:
fL = ?
(Foot-Lambert)
FL = 1/pi candela/ft^2
In retinal illumination, what is the quantity?
The amount of light falling on the retina
What 2 things tell what the illumination will be?
Luminance and pupil size
Units for retinal illumination?
Trolands (Td)
Formula for retinal illuminance
Td = (nits)(mm^2) Troland = (luminance of the object in nits)x(pupil area in mm^2)
What is unique about retinal illuminance as the object distance changes?
The retinal illuminance remains constant for any object distance
What is the difference for object distance between retinal illumination and the inverse square law?
RI: illumination remains constant for any distance
ISL: if distance to screen increases, then luminance decreases
If illumination entering the eye decreases with the square of the distance, how can retinal illumination remain constant for all distances?
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.
Suppose you are looking at a piece of paper in bright sunlight. What would the retinal illuminance be for the image of that paper on your retina, assuming the luminance of a piece of paper in sunlight is about 10,000 nits and your pupil diameter is equal to 2.0 mm?
Pupil area = (pi)r^2 = (pi)(1^2) = 3.14592 mm^2
(Lum nits)x(pupil mm^2) = (10,000 nits)x(3.14592 mm^2) = 31,415.92 Td
Using artificial light sources, which type of light is for longer wavelengths, and has a high performance? (It’s a filament that’s heated)
Incandescent
Using artificial light sources, which type of light has a behavior that is eradic for different wavelengths, showing spikes across the spectrum?
Fluorescent
Using artificial light sources, which type of light uses the blue spectral region?
LED
Theoretical construct that is convenient for describing sources of EMR, including light sources
Blackbody radiator
What does a blackbody radiator absorb and reflect?
- Absorbs all light/energy that’s thrown at it
- Reflects nothing
When talking about blackbody radiators and color temperature, as the temperature increases, what happens to the peak wavelength and the power (area under the spectral distribution curve)?
As temp inc, peak wavelength dec, and power inc
As you shift from 10,000K (kelvin) to 5,000K to 2,000K, when discussing blackbody radiators and color temp, what happens to the peak wavelength?
There’s a shift in the peak as temperature changes
A blackbody radiator with a temperature of 2,000K has most of it’s power concentrated at what wavelength?
Longer wavelengths
A blackbody radiator with a temperature of 10,000K has most of it’s power concentrated at what wavelength?
Shorter wavelengths
For blackbody radiators, most energy is concentrated in the shorter wavelength as the temp is __1__ and concentrated in the longer wavelength as the temp is __2__.
1: Higher
2: Lower
Which color temperature would appear yellow-white because of it’s relatively higher energy level in the long-wavelength region of the spectrum?
2,000K
Which color temperature would appear blue-white because of it’s relatively higher energy level in the short-wavelength region of the spectrum?
10,000K
Between a 4000K LED and a 2700 high pressure sodium bulb, which one allows us to see better at night?
4000K LED
What’s the disadvantage of being able to see with a 4,000K LED at night better than with a 2,700K bulb?
It has 3 times more scattering at 465nm than at 600nm (Rayleigh Scattering)
The higher the color temperature, what color is the light, how much scatter is there, and what’s the area of glare?
- Bluer the light
- More the light scatters
- The larger the area of glare
What kind of glare does higher color temperature cause?
Discomfort glare
Who does discomfort glare affect most?
Presbyopes and cataract patients
RGB cones response is from what 2 wavelengths?
400-700nm
RGB cones peak response is at what wavelength?
555nm
Rods response is from what 2 wavelengths?
400-600nm
Rods peak sensitivity is at what wavelength?
507nm
The peak luminous efficacy of photopic vision (day vision, where only cones are active) is what?
680 lu/W
The peak luminous efficacy of photopic vision (day vision, where only cones are active) has a peak response of what wavelength?
555nm
The peak luminous efficacy of scotopic vision (night vision, where only rods are active) is what?
1700 lu/W
The peak luminous efficacy of scotopic vision (night vision, where only rods are active) has a peak response of what wavelength?
507nm
Photopic vision
Day vision
Only cones active
Scotopic vision
Night vision
Only rods are active
Mesopic vision
Sundown (light/dark)
Both rods and cones are active
At mesopic and scotopic conditions, the eye is more sensitive to what wavelength of light, and what color temperature source?
Shorter wavelengths
Higher color temperature sources
A filter that passes only a certain specified spectral band of light
Bandpass filter
What filter is not based on color, but only wants a specific wavelength (ex: only b/w 500 and 550 nm)?
Bandpass filter
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 filter
A filter that transmits long wavelengths of light, but not shorter wavelengths
Long-pass filters
Filters that produce a very narrow band of light based on the interference principle of light
Interference filters
Color mixture with 1 long pass and 1 short pass filter, together in sequence
Subtractive color mixtures
Explain subtractive color mixtures
2 filter combination in sequence transmits less light than either one by itself
(Light source)–[lots of light = long wavelength]–>(filter #1)–[less light = shorter wavelength)–>(filter #2)–[even less light = shorter wavelength]
Color mixture with 2 different color filters, both projecting onto a screen
Additive color mixtures
Explain additive color mixtures
Light is projected through 2 different color filters and combined on a screen to produce an additive mixture. It has more light than either one by itself.
Filter that transmits all wavelengths equally and minimizes color distortion
Neutral density filter
Neutral density (ND) filters are specified by their what?
Optical density (OD)
How do you calculate the optical density for a neutral density filter?
OD = log(1/T)
OD is in log units
T is the fraction transmittance
Why are ND filter used in combination with high-pass filters?
Block UV radiation in sunglasses to minimize color distortion for patients who have anomalous (varying from the norm) color vision
Choosing an ND filter with a higher optical density will translate to ____ transmission and ____ absorption of the incident light.
Lower transmission
Greater absorption
For higher transmission and less absorption, a ____ optical density would be appropriate.
Lower optical density
Where is UVB absorbed?
By the skin epidermis
What does UVB absorption result in?
Sunburn
Absorption of high-energy radiation (UVB), can damage and lead to what?
Damage DNA, leading to skin cancer
How does early exposed to UVB rays affect children and adults?
Exposure early in life appears to be particularly harmful, predisposing a child to developing skin cancer later as an adult.
When is UVB present in North America?
During the summer months
Where is UVA absorbed?
By the skin epidermis and underlying dermis
Absorption of high-energy radiation (UVA) promotes what?
Wrinkling of skin
When is UVA present in North America?
During all four seasons
What are some important structures that are vulnerable to UV radiation?
Ocular tissues, as well as the adnexa (the areas around the ocular tissues)
Skin
Acute exposure to UVC can cause what?
Painful solar keratitis
When is UVC present, and when can you get painful solar keratitis?
When snow skiing without ocular protection
Chronic UV exposure appears to play a role in the development of what?
Pinguecula, pterygium, and certain cataracts and may contribute to the development of age-related macular degeneration (AMD)
