Why You Shouldn’t Use Solar Power Meters For Red Light Therapy Devices
Solar power meters are notoriously inaccurate for measuring light irradiance from red light therapy units, here’s proof.
Solar meters are notoriously inaccurate for measuring light irradiance from red light therapy units. Solar meters are broad-spectrum devices designed to measure sunlight, hence the name “Solar Power Meter”. Yet despite this fact these solar power meters are inaccurate for measuring narrow bandwidth light radiation from RLT units you see many authors and even manufacturers (still) using these devices and quoting mw/cm² power levels. Some try to justify their usage by saying that they know the meter is inaccurate, but they are using it to check relative power levels between RLT units.
For a short amount of time, I sold Red Light Therapy units. Before I sold a device, I made sure I obtained highly accurate light irradiance measurements from the RLT devices for my customers. After a great deal of research, I purchased an Apogee MQ-620. At the end of this article, there is a section on the accuracy of the Apogee MQ-620 meter. The accuracy is explained in detail for those who are interested.
Testing the Tenmars Solar Meter
Since solar meters are used and quoted so often, I decided to purchase a commonly used solar meter just to check its accuracy. I purchased the Tenmars Solar Power Meter TM-206.
When I began comparing 660nm light and 850 nm light radiation power measurements what I found is that the Tenmars solar power meter gave erroneous readings.
Typically, the Tenmars solar meters measured 660 nm light with a +40% increase of radiation reading from the actual power. But even that error paled in comparison to the error factor for 850 nm light. It measures 850 nm with a +100 % increase of radiation reading from the actual light radiation power. If we combine the error factors for both these wavelengths of light, the Tenmars solar meter is erroneously measuring the light radiation with a +90% error.
That’s great if you are a manufacturer wanting to promote the power of your RLT device. By using a Tenmars Solar Power meter to measure the light radiation, you can almost double its apparent power output. Is this why the meter is so popular?
To perform the test I used a 600-watt unit, 660 nm, and 850 nm RLT unit. This device has a switch-selectable for both wavelengths allowing me to test each wavelength independently of the other.
Power measurements were made at 6”, 12”, and 18” distance from the center of the device. The wavelengths were measured separately.
Confirming the Accuracy of the Apogee Meter
To confirm the Apogee MQ-620 meter was providing accurate measurements I sent my RLT device to LightLab International in Allentown PA. for photometric testing. The results are shown in the following table.
Putting It All Together
The following table illustrates all the data in one table.
The data confirms that Solar Power Meters are not an instrument to be used for measuring light radiation from RLT devices. I would question the integrity of any measurement that used solar power meters for analysis.
Accuracy of the Apogee PAR Meter
The MQ-620 an extended range PAR meter from Apogee Instruments. The meter can accurately read light irradiance from 340 nm to 1040 nm wavelength. The response of the light sensor is flat over these wavelengths, meaning it will accurately measure the photon flux, across these wavelengths.
However, the energy content of photons depends on their wavelengths. Photons with shorter wavelengths have higher energy content than those with longer wavelengths. This means that 1000 photons at a 450 nm wavelength, have more energy than 1000 photons at 900 nm.
To obtain an accurate mW/cm² we need to compensate the photon flux for the energy of the light based on its wavelength. That is the Energy Compensation by Wavelength, which is shown on the right side of the above graphs.
For the two wavelengths, we are measuring the compensation factors are shown below.
850nm Joules per umol 0.140737093
660nm Joules per umol 0.181252316
The measurement of the instrument is in umol/m² and is converted to w/m² using the conversion factor. Then we convert from w/m² to mW/cm². The accuracy of the meter is quoted at 5% to 10% of the actual value.
Who is Apogee Instruments?
Apogee Instruments was established 25 years ago. Their motto is “Built by scientists, for scientists.” Currently, Apogee Instruments manufacture numerous environmental instruments and sensors that are used globally.