Photometry VP

What is a troland?

The retinal illunimance produced by viewing a 1 cd/m^2 source through a 1 mm^2 pupil. Trolands = Luminance (cd/m^2) times pupil area (mm^2) 1 td = cd/m^2 * mm^2 Doesn't take into account media transparency!!

What does troland not take into account?

media transparency

A point source has a luminous intensity of 10 candelas. What are the units of total luminous power of the source?

Lumens

What is the difference between luminance and brightness? hmmm

Luminance is a physical property of how much is given off whereas brightness is perceptual However, the two are often related to one another.

What is difference b/w luminous power, luminous intensity and luminance?

Luminous Power (luminous flux) • Total power produced by a source in all directions • Units: Lumens (abbreviated lm) Luminous Intensity • luminous power produced in a solid angle (certain direction) by a point source • Units: Candelas (1 lumen/steradian (solid angle unit) (abbreviated cd) • Extra -Most non-point sources radiate different amounts of luminous energy in different directions • The most luminous power is coming straight down and it is lower in other directions. Luminance • Luminous intensity per projected area of an extended source • Refers to light coming off an extended source • Light coming off a surface. • Units: lumens/str/m2 = candelas/m2 Illuminance • Luminous power (not discriminate) falling on a surface • Units: lumens/square meter

What is the relationship between Lumens and Watts? Draw diagram

• *683 lumens*/W @ *555 nm* - for both photopic and scotopic conditions - 555 nm is the peak of our photopic luminosity • *Scotopic* is more sensitive to *shorter* wavelengths (<555) whereas *photopic* is better at *longer*

How many lumens are there in a light that contains 10 W of 555 nm plus 15 W of 600 nm (spec sens .63) ? What principle are we assuming?

• 10 watts at 555nm = 6830 lumens (1 watt = 683 lumens) • To find at 600 nm, multiply 683 by .63 and then by 15 • Total lumens: 13284 lumens assuming that these are additive. • For photometry to make sense we have to be able to add up the lumens of each individual wavelength this is the principle we assume

Does luminance change with distance?

No - as distance increases, you have more point sources but each point source is reduced in a relative way that leads to same luminance (density more diffuse)

A point source has a luminous intensity of 10 candelas. What is the total luminous power of the source if it radiates equally in all directions?

Since per 1 candela there are 4π lumens, there will be 40π

What is the retinal illuminance when viewing a 400 cd/m2 wall with a 5 mm diameter pupil? (radius = 2.5mm)

T = L x A Area of a circle = π (r^2) So 400 cd/m2 x (2.5^2) = 7854 trolands

What is a cosine diffuser? What relates?

• A special kind of surface whose luminance does not vary with the angle at which it is viewed • As you go away from the perpendicular the intensity of the point will reduce with the cosine of the angle. • Area also reduces as the cosine of the angle, so luminance (which is intensity per unit area) remains constant • Because perfect reflectance - Illuminance 1 lumen/ft2 reflect to Luminance 1/π candelas/ft^2 = 1 foot-lambert

Explain the inverse square law. Included for tilted surface.

• E = I/d^2; E is illuminance (lux = lm/m2); I is intensity (cd, or lumens/str); d is distance in m • you will reduce the illuminance on a surface by a factor of 4 as distance doubles. E = [I/d2]. cos θ θ is the angle from the perpendicular cos 0° = 1, cos 90°=0 • If source is tilted, fewer lumens fall on it and less illumination

Two lights contains equal energy at 555 nm and 507 nm. What statement regarding photopic and scotopic lumens would be true?

• The radiant power for both is the same. • However, for luminous, the 507 produces more scotopic lumens of light than the 555 nm scoptic. • 555 produces equal numbers of light both scotopically and photopically.

What watt is what?

• When we talk of artificial light sources we often speak of electrical efficiency, i.e. how many Watts of electricity are needed for a particular bulb to produce a lumen • However, the light/radiation produced has radiant power (energy per sec), which is also in Watts • The electrical Watts and radiant power Watts are not the same • To get 1 Watt of 555 nm light, we need lots of quanta! • (Actually, using Energy=hc/λ, each quantum has ~3.6x10-19 J, so we need ~3 x 10 18 quanta/s to get a Watt)

What is a steradian?

• conic section of a sphere • Steradians = Area (mm sq.)/ radius^2 (mm^2) • There are 4π steradian in a sphere • 1 candela has 4π power of lumens; so 4π lumens per candela

What is the cornerstone of photometry? how is it different from radiometry?

• photo - The eye's response to electromagnetic radiation • radio - how much the EM source actually produces; radiant power • Lumens - the RATE of delivery of luminous energy; measurement of luminous power • Watt - RATE of delivery of radiant energy • luminous power - used to talk about light that emerges in all directions from a source; luminosity is the "visual effect" but might actually have same amount of energy