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Home / Technical Articles / Luminous Measurement Graphic Representation
Goniophotometer - used to determine the luminous intensity of a source of light in all spatial directions in relation to a vertical axis.

The collection of luminous intensity emitted by a source of light in all directions is known asluminous distribution. The sources of light used in practice have a more or less large luminous surface, whose radiation intensity is affected by the construction of the s ource itself, presenting various values in these scattered directions.

Special devices (like the Goniophotometer) are constructed to determine the luminous intensity of a source of light in all spatial directions in relation to a vertical axis. If luminous intensity (I) of a source of light is represented by vectors in the infinite spatial directions, a volume representing the value for the total flux emitted by the source is created.

Such a value may be defined by the formula below:

Formula - Total flux emitted by the source is created

Photometric solid is the solid obtained. Fig. 1 shows an incasdescent lamp photometric solid.

Figure 1 - Incandescent lamp photometric solid
Figure 1 - Incandescent lamp photometric solid

If a plane passes through the symmetric axis of a source of light, for example, a meridional plane, a section limited by a curve, known as photometric curve, or luminous distribution curve is obtained (See Figure 2).

Figure 2 - Photometric curve for an incandescent lamp.
Figure 2 - Photometric curve for an incandescent lamp.

By reviewing the photometric curve of a source of light, luminous intensity in any direction may be determined very accurately. This data are necessary for some lighting calculations. Therefore, spatial directions through which luminous radiation is irradiated may be established by two coordinates.

One of the most frequently used coordinate systems to obtain photometric curves is the “C – y” represented in Fig. 3.

Figure 3 - C - y coordinate system
Figure 3 - C - y coordinate system

Photometric curves refer to an emitted luminous flux of 1 000 lm. Generally speaking, the source of light emits a larger flux. Thus, the corresponding luminous intensity values are calculated by a simple ratio.

When a lamp is housed in a reflector, its flux is distorted, producing a volume with a marked shape defined by the characterist ics of the reflector. Therefore, distribution curves vary according to different planes. The two following figures show two examples where distribution curves for two reflectors are represented.

Fig.4 reflector is symmetric and has identical curves for any of the meridional planes. This is the reason why a sole curve is enough for its photometric identification.

Fig. 5 reflector is asymmetric and each plane has a different curve. All planes must be known.

Symmetric photometric distribution curve and asymmetric photometric distribution curve.
Figure 4 (left) - Symmetric photometric distribution curve; Figure 5 (right) - Asymmetric photometric distribution curve.

Another method to represent luminous flux distribution is the isocandela curve diagram (Fig. 12). According to this diagram, luminaires are supposed to be in the center of a sphere where exterior surface points with the same intensity are linked (isocandela curves).

Generally, luminaires have, at least, one symmetric plane. This is the reason why they are only represented in a hemisphere.

Figure 6 - Isocandela curves
Figure 6 - Isocandela curves

This representation is very comprehensive. However, more experience is needed to interpret it.

The flux emitted by a source of light provides surface lighting (illuminance) whose values are measured in lux. If those values are projected on the same plane and a line links the ones with the same value, isolux curves are formed (Fig. 7).

Figure 7 - Isolux curves
Figure 7 - Isolux curves

Finally, luminance depends on the luminous flux reflected by a surface in the observer’s direction. Values are measured in candelas per square metre (cd/m2) and are represented by isoluminance curves (Fig. 8)

Figure 8 - Isoluminance curves
Figure 8 - Isoluminance curves

Luminous measurement summary chart

Chart 1. Luminous measurement summary
Chart 1. Luminous measurement summary

SOURCE: Lighting Engineering 2002 – Indalux

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

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