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The following discussion covers three main topics - (1) Light Measurement; (2) Reflected vs. Incident Light; and (3) Light Meters.
Light Measurement
The Instruments that measure light are measuring two things: the Quantity and Quality of light.
Quantity is defined as a measure of a certain light source or an area. This can be applied in a variety of ways, from compliance of OSHA safety requirement in a factory to light level measurement around a bank ATM machine (for security reasons) to a studio set in Hollywood for filming. Light is measured in foot candles (English measurement) or Lux (Metric measurement).
Quality relates to the color of the light sources - (color temperature reading in Kelvins Degrees). For example, twelve o'clock noon sun on a clear day in California would normally be the ideal condition for filming. The Kelvins reading would be approximately 6500 (United States broadcast standard).
Reflected Light vs. Incident Light
Photographic exposure may be based on the measurement of either light falling on a subject or light reflected from it. The first method is called INCIDENT light measurement and the latter REFLECTED light measurement.
For many years, the reflected-light method was popular with photographers, amateurs and professionals. But knowledgeable professionals soon realized that exposures based on reflected-light were seriously in error whenever the subject was predominantly either dark or light. While these errors were not too apparent in black-and-white, because of the wide latitude of the film, they could be disastrous in color photography.
Reflected Light
Most scenes contain a full range of tones with reflectance values varying through a wide range; black may have a 2% reflectance value and white may be up around 95%, the numerous other tones somewhere in between.
Let's take an example of a scene containing both pure white and pure black. On a reflected-light meter, the black will give a low reading and the white a high reading. Which is correct? If two separate exposures are made, one for the low and one for the high reading, both tones will be represented by the same middle density on the film in the resulting reproductions. Both black and white tones will look alike. In fact, any tone measured by the reflected-light method will be represented by the same middle density!. Obviously, both exposures are incorrect - one over exposed, the other under-exposed.
This example, simplified for the sake of brevity, points out the shortcomings of the reflected-light meters. Also, the "angle of acceptance" of the reflected-light meter has always been a problem. A bright background such as the sky or a bright window can cause erroneous readings. The same is true also of dark backgrounds such as dark green foliage. The photographer working with such a meter adjusted for these limitations, developing methods of integrating several readings and making compromises between the meter readings and his/her own experience. In short, the photographer had to "live with it".
With the increased used of color photography came the need for more critical exposure. As more photographers became dissatisfied with the reflected-light method, the "grey card technique" was introduced to help overcome the inherent errors in reflected-light meters. The grey card, with a standard reflectance value, would help the reflected-light meter measure the light falling on the subject. Though paradoxical, this is, in effect the principle of incident-light measurement.
Incident-Light
Regardless of the number of tones in a scene or their reflectance values, or whether it is high-key or low-key, the exposure required to reproduce the tones correctly is determined by the amount of light falling on the scene. Even when the tones within the scene change, the exposure remains the same providing the light incident on the scene remains the same.
Thus, referring back to the example where a scene has a black tone of 2%, reflectance and a white tone of 95% reflectance, an incident-light meter would have indicated the correct exposure in one reading. The resulting reproductions would have placed the black tone at the lower end of the D. log_E curve and the white tone at the top of the curve.
Besides accuracy, the incident-light meter offers many other advantages. The amount of light incident upon a scene will always be greater than that reflected from it. As a result, the incident-light meter will have greater sensitivity than the reflected-light meter. Also, incident-light meters are more robust and reliable in practice than reflected-light meters of the same sensitivity. Another advantage is that the incident-light meter user rarely has to be concerned with the acceptance angle of the meter. It is interesting to note too, that most major film manufacturers use incident-light measurements for their published standards for correct exposure on their data sheets.
The SPECTRA CINE incident-light meters are each fitted with a hemisphere over the photocell. This hemisphere (called a Photosphere) is an almost perfect duplicate of a subject. It receives light from the same direction as does the subject and the meter gives a true reading of all useful light - key, fill and background.
Exposure Meters
There are primarily three types of exposure meters: incident type (ambient and/or flash), reflected (spotmeter, one degree) and color temperature.
- 1. INCIDENT TYPE EXPOSURE METERS
Initially developed by Karl Freund and Don Norwood (see Spectra History), the incident type exposure meter measures the intensity of the light falling (or incident) upon the subject. A hemispherical diffuser in front of the cell integrates the light falling from different directions. The incident meter recommends the correct exposure for an average, three-dimensional subject in that light.
The incident meter first gained popularity in the movie industry. Movie makers needed a meter that would give a constant interpretation from scene to scene of the skin tone of the leading actors - regardless of background or the amount of backlight used on the sets.
So many cameras have excellent built-in meters today that, for many photographers, the thought of owning another hand-held meter might seem odd. Nonetheless, built-in camera meters are generally the reflected-light type. As any professional photographer will tell you, there are many situations that can baffle and confuse even the most advanced reflected-light meters. In fact, there are many important lighting situations that are simply beyond the scope of any built-in meter.
Today, the incident meter is popular with any photographer (still or cine) who has to work in a hurry without evaluating the effect of the relatively unimportant surroundings or misleading subject or background brightness on his exposure. The incident meter is highly important and practical meter.
- 2. SPOT METERS
The one degree spotmeter is the most popular reflected type meter. The one degree spot has the ability to reach out and confine its reading to important subjects at a distance from the camera. Whether it be a spotlight figure on the stage, the speaker at a news conference, or an animal at the zoo - often the only way to meter an unapproachable subject accurately, especially if the surroundings are misleading, is to use the spotmeter.
- 3. COLOR METERS
The color temperature meter is used to measure the precise color temperature of any light sources (quality of light sources). It then recommends the most appropriate correction filters (light balancing or color compensation filters) to balance the color with the film type set on the meter. The best meter to use for cine or still photography is the three color temperature meter.
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