Chapter 3

Measuring / Testing Optoelectric Devices

• Luminous Intensity• 1 candela (cd) = 4 π lumens (lm)

• Equivalent to one lumen per steradian (1 lm/sr)

• Data Sheet for LED

• Red LED has Iv = 70 mcd (millicandelas)• How many lumens is this?

• 70 mcd = 0.88 lm

• Angle of Half Intensity• Half power

cd

lm

mcd

cd

_1

_4

_1000

_1

LEDs

Light Intensity Meter

Light intensity meter receives unfiltered light that does not correspond to response of human eye.

Color Temperature Meter

Color

Exposure Value

Exposure Value

Aperture Value

Time Value

Speed Value

Brightness ValueBfl is the illuminated object's

brightness in foot-Lamberts

Exposure Value

Can make Exposure Value equal to Brightness Value if the Speed Value is zero.

If the Speed Value (Sv) is zero.

If S equals 3.3, then Sv is zero.

If S equals 3.1, then Sv is close to zero,which corresponds to ASA(ISO) of 3.1

Can then use the Exposure Value from the meter directly to find Brightness Value

Lambert's Law or Law of Cosines

detector.

Spectroradiometer

A spectroradiometer is also called a scanning colorimeter.

It does a lot of number crunching to determine the amount of light at each wavelength.

Red LED

Problem with measuring LEDs is the light energy usually peaks in the red region of the light spectrum.Standard sensor does not match the CIE curve.Need to use filter for the specific LEDs wavelength.

Reflectance

Dark paper has low reflectance. Light paper has high reflectance.

Divide luminous reading in foot-Lamberts by illuminance in footcandles to derive reflectance factor.

Luminance = Illuminance X Reflectance Factor

Transmissivity

Transmissivity - the ratio of light which passes through a material to the light incident upon its surface.

Index of Refraction

Describes relative transmittance of optical filter with respect to its wavelength.

Determines amount of incident light reflected at the filter/air interface.

Plastic filter with a refraction index of 1.5 reflects 4% of the light at the interface when the light is normal or perpendicular to the filter.

Photometer should have color response as close to the CIE curve as possible. Silicon photodiode with glass filters provide a better CIE curve match than selenium and cadmium cells.

Numerical aperture (NA) refers to the sine of the half angle of the acceptance cone of the light.

Characterizes the range of angles over which the system can accept or emit light.

Exposure Time

f value of a lens – the ratio of its clear or opened aperture to the focal length

where k = 3.33 (constant)

N is the f-stop B is the scene illumination (foot-Lamberts)

S is ASA (ISO) rating of film

t is exposure time

t is the interval the shutter remains open in seconds

BS

kNt

2

Greyscale

A grayscale is a chart of progressively darker shades of gray placed next to one another.

You use the greyscale to determine how much contrast the camera delivers.