PRODUCT REVIEW		RICHARD HARKER

An Inexpensive Light Meter and Its Application to Reefkeeping

RICHARD HARKER

The lux meter reviewed in this month’s column is available from several different sources and is listed simply as a “lux meter,” with no brand name assignation.

The importance of adequate lighting over a reef tank is well documented, but determining light levels over the tank has been beyond the reach of the average hobbyist. Nearly all contemporary light measurements of corals and coral reefs are reported in “photosynthetically active radiation” (PAR). Meters that measure PAR can run anywhere from several hundred to several thousand dollars. Inexpensive light meters available to the hobbyist measure light in units of lux, a photometric unit more appropriate for photography and interior design. Until now, it has been difficult for a hobbyist to compare light measurements using one of the inexpensive lux meters to measurements in PAR.

To see if an inexpensive lux meter could be of value to a reef hobbyist, I compared a reasonably priced lux meter to a PAR meter and developed a formula that will enable a hobbyist to compare his or her tank lighting to published light measurements over coral reefs. The lux meter I examined is available from different sources for a little over $100 and carries no brand label. In catalogs, it is listed simply as a lux meter. The unit can read up to 50,000 lux using three switchable scales, and uses a remote sensor that is reasonably water resistant.

Do you have questions, want to discuss the issues raised in this column, or read the comments of other aquarists and the answers from columnist Richard Harker? You can do so by going to Product Interactive.

To measure the performance of the lux meter, I compared it to a LiCor LI-1000 Datalogger using a 2pi PAR sensor and a Gossen 3C lux meter. The light sources I used for the measurements were 10 popular metal halide bulbs that I had used in previous studies. The bulbs were allowed to warm up until light levels stabilized and then a reading was made using the PAR meter. The sensor of the lux meter was then placed in front of the PAR sensor and a reading taken. Lastly, a reading was taken with the Gossen lux meter.

The results of the measurements are shown in Figure 1. For the most part, the ratio between the PAR meter and the lux meter is constant across bulbs. In other words, as PAR goes up, lux as measured by the meter goes up at the same rate. (The 400-watt Iwasaki bulb generates so much light compared to the other bulbs that it is well off the scale. Its performance is shown in the table below.)

Figure 1

Regression analysis is a statistical tool that examines the relationship between two variables and tells us how they are related. Linear regression applied to PAR and lux suggests a strong linear relationship with an R-squared of 95.3 percent. This means that over 95 percent of the change in PAR from bulb to bulb can be explained by the change in lux. In other words, using the lux meter and calculating PAR closely approximates what a considerably more expensive PAR meter would read. The regression analysis also provides a formula to calculate PAR from lux, which is:

PAR = 1.53 + (0.0111)lux

As an alternative to using the regression formula, one can directly apply the factor shown in Table I. It is the number by which one multiplies the lux reading to approximate the PAR of a specific bulb.

As the table demonstrates, the higher color temperature Coralife bulbs tend to cluster together with very similar factors. These bulbs are extremely blue, as is the Belgium 14 Kelvin (K) bulb. The Iwasaki 6500 K bulbs in both wattages have essentially identical factors. The 400-watt Radium 20 K, a German bulb, and the Hamilton 175-watt 10 K bulb, another German bulb, have identical factors. This suggests that brand and origin of manufacture are more important than wattage when choosing a factor to estimate PAR.

Figure 2

Figure 2 shows how the lux meter compares to a Gossen 3C professional lux meter used in photography. The inexpensive lux meter tends to be less sensitive to color temperature than the Gossen meter. This suggests that the meter does not conform to the Commission Internationale de I’Eclairage (CIE) Standard Observer Curve or photopic curve. This is a weighting curve applied to photometric sensors so that the sensor measures light as a person would see it. Our eyes are more sensitive to yellow and green, so greater weight is given to light in the 500 to 600 nanometer wavelength range. This weighting produces higher lux readings for lower color temperature lighting.

For example, the Coralife 6500 and 10,000 K bulbs produce very similar PAR — 26.5 microEinsteins per square meter per second (µE/m²/sec) versus 24.9. Because of photometric weighting, the 6500 K bulb produces over twice as much lux (3100 versus 1300) as measured by the Gossen meter. The difference is not as great with the inexpensive meter. This is one rare example where cutting corners to keep costs down has inadvertently benefited the reefkeeper. Presumably to save the cost, the manufacturer has not incorporated the correct photopic curve, consequently giving greater weight to shorter wavelength light than a more expensive lux meter.

The greatest benefit to owning any light meter is the ability to track lighting intensity over time. Whether one uses metal halide or fluorescent lamps, the lights will begin to deteriorate the moment they are installed. Because our eyes are very adept at adapting to changing light levels, using our impression of light levels to determine when to replace lights can be very misleading. A better way is to use a meter to measure light levels with new lamps and then again measure light levels each month until you replace the lamps. Tracking the decline in light intensity over time, combined with watching how your animals respond to the diminishing light, will give you a sense of how often you should replace the lamps.