Subject reflectance is thus assumed to be equal to the ratio of the exposure constants K and C. For example, a meter may be calibrated for a midtone so that K = 3.64 and C = 25.6, so that R is approximately 0.14 or 14 per cent. Other values such as the 18 per cent reflectance of a neutral grey test card are used as a substitute 'average subject'. The reasoning for use of an incident light reading of subject illuminance ES is that by definition no subject can have a reflectance greater than 1.0, and exposure may be adjusted so that the brightest highlight renders an acceptable maximum or minimum density in a negative or transparency respectively. But since in equation (10) the illumination ES is not precisely defined as to source size or direction, the light meter must integrate the incident light over a large acceptance angle to account for all contributing sources. For this purpose a translucent diffuser is placed over the photocell for incident light readings. The diffuser also reduces the response of the photocell, and different transmittance values or masks may be used to limit the reading with intense light sources such as studio flash units.
The diffuser may be flat or dome-shaped to give different forms of response. For a flat diffuser, if the angle between the normal to the photocell surface and
The flat cosine receptor is directional and necessary when illuminance measurements are to be made taking into account Lambert's Cosine Law. However, when a flat diffuser is used for making measurements of illuminance for a photographic exposure, the meter is pointed in turn at the camera position from the subject, then at the brightest source of illumination from the subject (Figure 19.6). The log arithmetic mean of the two readings is then taken. This average is the mid-point of the readings on the (logarithmic) scale of the meter. This technique is called the Duplex method. Alternative methods include either pointing the meter towards the camera, or, pointing midway between the camera and a single illuminant, or taking averages of readings towards the camera and each illuminant in turn.
The lighting ratio, based upon the measurement at the subject with the meter pointed at each illuminant in turn, requires the much more directional response of a flat diffuser. For this reason many flash exposure meters are fitted with a flat diffuser. When an exposure meter is converted for use as a photometer to measure illuminance on a surface, again a flat diffuser is essential.
A more convenient method of incident-light metering is to use a diffuser of hemispherical shape; in this case the meter is simply directed from the subject towards the camera position for a single measurement. Its shape effectively presents a surface that is equally oriented in all directions relative to the subject-camera axis and its projected area relative to a beam of light at any angle of incidence is the same.
The compact invercone design of diffuser as used on the Weston series of exposure meters was a truncated hemisphere with an inverted cone inside it. It overhung the meter body and picked up any significant backlighting on the subject. The term 'invercone' is often generally applied to diffusers for meters.
a ray of light from the primary illuminant is denoted by 9, then the response (r) is given by ka cos 9
and of a hemispherical diffuser by kb (1 + cos 9)
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