User Manual

5. MEASURE TECHNIQUE

Illuminance - foot-candles or lux

This is the amount of light energy incident upon a plane. The plane is defined by the geometric plane of the

white disc on the turret. Light sources within the (180°) hemisphere in front of the white disc will be sensed

according to the cosine law.

For best accuracy do not block light sources in the 180° hemisphere. The orientation of the white disc is

designed to minimize the effect of the operator's presence on the measurement.

COSINE LAW: RELATIVE SENSITIVITY

VERSUS ANGLE OF INCIDENT LIGHT

Luminance - foot-lamberts or nits (candela/m

)

Luminance is the photometric brightness of sources or light reflected from objects. Photo-Meters read the

overall luminance in approximately a 25° field of view, as shown.

RELATIVE SENSITIVITY

VERSUS ANGLE

When attempting luminance readings avoid casting shadows upon the subjects when those are reflecting

objects (i.e., not light sources).

6. MISCELLANEOUS

Photo-Meters are very sensitive instruments that respond to slight changes in illumination. This is especially

true when measuring fluorescent lights.

Fluorescent illumination varies in brightness at twice the rate of the AC line frequency (60 hz). This variation

is not apparent to the human eye, but it will be apparent in readings with Photo-Meters. A slight drift in the

reading of the instruments might indicate the condition of fluorescent illumination. The average of the highest