Lighting Technology: Basics and Characteristics (1)

Anyone who is interested in lighting technology and lighting design should be familiar with the basic properties and characteristics of light.

These differ in quantitative properties - i.e. quantity of light, luminous intensity, etc. - and in the qualitative composition of the light - i.e. topics such as light spectrum, colour temperature, colour rendering, etc. 

This article describes the quantitative characteristics. We will not go too deeply into the physical basics, but only consider what is relevant for the work at Film & TV.

Lamps and luminaires

We generally distinguish between the terms lamp and luminaire. A lamp is the component that produces the light. For example, an incandescent lamp. In the film and television industry, the term burner is used instead of lamp, e.g. for a 1000 watt halogen burner.

The complete "device", i.e. with power cable, switch, housing, reflector, lenses, etc., is called a lamp. Thus there are no film or TV lamps but only film and TV lamps or also film and TV spotlights. Spotlights are a subcategory of luminaires that can focus light more strongly.

The basic units: Lumen, Candela and Lux

Luminous flux, measured in lumens

Lumen is the luminous flux of a lamp. This describes all the light emitted by a lamp. Independent of the direction of light or light concentration. All the light that is emitted is completely specified. A luminous flux can also be specified for complete luminaires, although this is more common for general luminaires (office, industry, etc.).

The luminous flux can usually only be measured in a laboratory. The lamps (or entire luminaires) must be used in integrating spheres for this purpose. Each manufacturer, however, specifies the luminous flux of his lamp so that this can be compared.

Luminous intensity, measured in candela

The candela unit indicates the luminous intensity of a luminaire. Unlike luminous flux, luminous intensity is directional. Almost all luminaires have a specific main direction of light emission. This is why luminous intensity is often shown in diagrams.

The green graph shows a very tightly focused TV spotlight with high luminous intensity in the centre of the light cone (0°). The headlight with the blue graph scatters the light much wider and has a lower luminous intensity. Both headlamps in the example have exactly the same luminous flux in lumen. The green headlight concentrates this light much more.

The luminous intensity is measured in lighting laboratories using a goniometer that rotates around the luminaire in all directions and records the luminous intensity.

Illuminance, measured in lux

While luminous flux and luminous intensity play a role when selecting spotlights, illuminance in lux is the most important parameter when working in a studio or on a set. The light that arrives is measured in lux. For example, the light that reaches a performer or newscaster. In TV studios there are often exact specifications, such as 300, 600 or 800 lux on the face of the presenter.

Illuminance depends, of course, on the luminous intensity and the distance from the spotlight. There is a correlation between luminous intensity and illuminance:

Luminous intensity in candela = illuminance in lux at a distance of 1m

However, the illuminance does not simply decrease linearly with increasing distance, but as a square! This means that if the distance doubles to 2m, only a quarter of the illuminance is achieved. At 3m, only one ninth of the illuminance is left, and so on.

This so-called photometric distance set can be very helpful in lighting design. Thus, an actor can be well illuminated without the - possibly unsightly - background receiving too much light. The distance between luminaire and performer only has to be significantly smaller than the distance between luminaire and background. In most cases, it is sufficient to position the performer slightly away from the background and move the luminaire(s) as close as possible to the performer.

Figure 1: Lighting design with little light in the background (source: Dennis Jackstien)

The illuminance can be determined on the set or in the studio with a commercially available light meter. With the corresponding settings of the camera (ISO, frame rate, shutter angle), the light meter can also directly output aperture values for the camera instead of lux.

Luminance, measured in candela/m²

Luminance, the last important basic parameter, is what our eyes or even the camera sensor perceives as brightness. How bright a certain area in the image looks depends not only on the illuminance of that area, but also on the texture and reflective properties of the area. A bright face reflects more light than a dark one.

Luminance is given in candelas per square metre or cd/m² for short. It can be measured on the set with a spotmeter from the camera direction. This allows image areas to be quickly checked for overexposure or underexposure without requiring the camera.

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