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Meter candle (mc.) is a unit of illumination equal to one lumen per square meter.
Foot candle (fc.) is the unit of illumination equal to one lumen per square foot.
8. BRIGHTNESS (B) of an element of luminous surface may be expressed in either of two ways (a) in terms of intensity (I), or (b) in terms of flux (F).
(a) Brightness in terms of the luminous intensity, I, (candle power) per unit of projected area of the surface (candle power brightness) corresponds to the defining equation
bi dl
dS cos 0 ^^
where 6 is the angle between the normal to the surface and the line of sight.
(b) Brightness in terms of the flux (F) preceding from a unit area of the surface, on the assumption that the surface is a perfect diffuser (i. e., that it obeys the cosine law of emission or reflection) corresponds to the defining equation :
^ = ^ (2)
F dS ^^
Lambert is a unit of brightness in the lumen system and is the brightness of a perfectly diffusing surface emitting or reflecting 1 lumen per square centimeter. For most purposes the millilambert (ml), 0.001 lambert, is the preferable practical unit.
9. LUMINOUS EFFICIENCY (C) of any source is the ratio of the luminous flux to the radiant flux from the source and is expressed in lumens per watt.
10. MECHANICAL EQUIVALENT OF LIGHT is the ratio of radiant flux to luminous flux for the wavelength of maximum visibility, and is expressed in ergs per second per lumen, or in watts per lumen. It is the reciprocal of the maximum visibility.
11. REFLECTION FACTOR (R) of a body is the ratio of the flux reflected to the flux incident. The reflection may be regular, diffuse, or mixed. In regular reflection the flux is reflected at an angle of reflection equal to the angle of incidence. In diffuse reflection the flux is reflected in all directions. In the case of perfectly diffuse reflection, the distribution of the reflected flux is in accordance with Lambert's cosine law. In most practical cases there is a superposition of regular and diffuse reflection.
12. ABSORPTION FACTOR (Ab) of a body is the ratio of the flux absorbed by the body to the flux incident upon it.
13. TRANSMISSION FACTOR (T) is the ratio of the flux transmitted to the flux incident.
Photographic Units.
In considering photagraphic materials it is convenient to regard them as light sensitive receptors similar in many ways to the
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