How to estimate the total luminous flux of a luminaire

If a fixture claims to be green and efficient, then you need to use a suitable method to verify that it is. Regrettably, the author can guarantee that in today's market, many so-called green products are actually not the case.

Forget the gorgeous rhetoric, the truth is in the numbers. But what if a manufacturer doesn't give complete data? If the reader has at least data about the center illuminance of the luminaire beam and the diameter of the illuminated area, you can still do something.

Figure 1 Three possible beam profiles

First, let's review the total luminous flux. Figure 1 illustrates the profiles of three possible beam profiles: A is a hypothetical fully flat beam profile, B is a peak-type beam profile that is often presented in an ellipsoidal spotlight, and C has a central dimple and a straight side. The beam distribution, which is very rare, is sometimes present in Fresnel lens concentrating fixtures. All three beam shapes have exactly the same central illuminance data, as shown by the red dashed line in the figure. This means that the photometers are placed at the three points a, b, and c of the beam, and their readings are the same.

Then, the central illumination (measured in footcandle or lux) does not indicate how much lumens are from the total luminous flux of the luminaire. To do this, you must have a corresponding illuminance reading across the beam. If you compare the beam pattern to a water bucket. The second row of beam profiles in Figure 1 are identical to the first row but are completely inverted, forming a U-shape. The last row is a water storage bucket that is constructed by rotating a U-shaped line. The straight beam A is constructed as a cylindrical bucket, while the other two beams B and C are constructed as more complex buckets.

If these imaginary buckets are filled with water, then the amount of water each bucket can hold can be compared to the total lumens of the luminaire's light output. (For example, bucket A has more water than bucket B.) Lumen number is the measured value of total luminous flux, not the measured value of light intensity, so a large and shallow bucket may have a narrow and deep water volume. The bucket has as much water. Similarly, a luminaire with a broad, narrow peak light distribution actually has as much light output as a narrower, larger peak light distribution luminaire. This is because the lumens of light are spread over a wider area.

If all the data is only a data chart of central illumination and spot diameter, how do you estimate the total luminous flux of the fixture?

The performance data of the V-shaped chart of Figure 2 is very common. (Note: This technique is only available for luminaires with circular, symmetrical beams, not for illuminators with asymmetrical beams, such as skylights.)

This table and graph shows the spot diameter and center illuminance over a range of shots. The author deliberately omitted the units of each physical quantity in this chart. As long as the units of these physical quantities are related to each other, whether in meters (m) and lux, or in feet and å‘Ž candlelight, it is applicable to the author's subsequent explanation. That is, if the range and diameter are in meters, the unit of illumination should be lux, and if the unit of the range and diameter is feet, then the unit of illumination should be lighted by candle.

Although the outline of the luminaire beam is not known in advance (the peak distribution is evenly distributed), some assumptions can be made to obtain a method of how much lumens the luminaire light output has. First, assume that there is no light distribution as in Fig. 1C - a small concave in the center. That is a special light distribution that is not common in conventional luminaires, so it is reasonable to exclude it. At this point, the light distribution is either a completely flat type light distribution as shown in FIG. 1A, or more likely to be a peak type light distribution, the center illumination is the highest, and the rear light level is gradually decreased toward the edge of the spot, as shown in FIG. 1B.

Thus, in both light distributions, a perfectly flat beam has the most lumens, and if its total luminous flux can be estimated, then the maximum lumen flux of the total luminous flux produced by such a luminaire is grasped.