I have been measuring the light spectrum (about 10 different wave lengths: 504 to about 1000 nm) and intensity at each of those wave lengths of two different 18W LED tubes: cool white, and warm white used for growing plants. The spectra distribution was the same (ie. ratios the same) but the intensity (mW/m2) slightly less for the warm white tube. According to the literature the warm white light should have relatively more red in the spectrum than the cool white light, but I could not detect such a difference. Any idea why that could be the case? I used a Newport Power meter model 1918-C for these measurements.
Thank you Georg,
I'm not using any monochromator. I locate the power meter directly underneath the light tube (and at different distances), and switch to the different wave lengths and obtain an intensity reading for that selected wave length.
Georg and Philipp are right;
please use a spectrometer. The expected differences in the spectra are small but the visual 'feeling' of cool white versus and warm white is quite strong.
The wave length switching of your power meter corrects the wave length dependent detection sensitivity of the detector, but prerequisite is that your detector gets monochromatic light according to that wave length.
You may use narrowband optical filters in order to provide monochromatic (or quasi monochromatic) light onto your detector and compare the results from your two light sources.
Agree with all. Please Do UV-Vis Spectrometer and use cutoff filters..
Thank you all for your very valuable contributions. It is much appreciated.
Another thing to consider is that the wavelength region from about 500-1000 nm might already be in the "warm" region of the spectrum where the spectral distribution is dominated by phosphor emissions. If the higher-CCT LEDs have a higher "spike" of short-wavelength energy near 450 nm from a blue-LED pump, you might be missing that portion of the spectral distribution.
The LED tubes you are comparing both likely have a Color Rendering Index of 70 CRI. The major difference is on the side of the spectrum which you did not measure (400~500nm). The LED tubes are probably made with pure blue LEDs and yellow YAG phosphor. The higher end of the spectrum (yellow-orange) could be the same, and the manufacturer adjusted the CCT by playing around at the blue end of the spectrum. The 6000K lamps have plenty of electrical power (optical power) in the blue region. That's why they measure higher intensity.
Most LED tube makers are not using red or yellow-orange phosphor (it's more expensive and doesn't last as long). Another issue you might face is spectral shift. What you measure today, may not be the same the lamp emits next month. While everyone touts 50,000-hrs or more of LED chip life, depending on what chip, what phosphor, and how it's driven, will affect the spectral output over time.
Not to sound like a commercial, you can see our background is in lighting, for commercial/industrial, for horticulture and in applied optics of light. If you're looking to grow plants indoors, we have both HID and T5 Fluorescent lamps that should satisfy almost any kind of grow test. For growing information, check out our website : www.eyehortilux.com. If you're into commercial/industrial lighting or solar/UV applications, we have other business-focused websites.
You need to use a spectroradiometer to see what you are looking for. Also, beware of using Lux values as they are weighted to the response of the eye.
If the spectra are the same between 500 and 1000 nm, both LEDs use the same phosphor. What differs is the proportion of blue light in the two light tubes. Probably the 6000K has less phosphor, the 3000K more, but they are basically the same hardware. Anyway you need the range 400~700nm to measure a visible light source. What you are searching for is out of the scope of your instrument.
- Badges
- Science topic