# The Difference between SPD and SQD spectra When we put a LED grow light into the integrating sphere test, we will see that there are two kinds of spectra. What is the reason for this? And which one should be used by cannabis cultivators?

The unit of SPD is mW/nm, the unit of SQD is umol/s/nm, umol/s is the unit of PPF (Photosynthetic Photon Flux). From this point of view, must look at the spectra of SQD.

Then why is there a difference in the spectra when the units tested are different?

The denominator is n*h*c, where n is the number of 1 mol photons, h is Planck’s constant, c is the speed of light, all are constant values, the result of nhc multiplication is 120. Thus, we can know that the PPF is determined by Pw and λ (wavelength), where Pw is the radiation flux, so when the radiation flux is constant, the larger the λ, the larger the corresponding PPF will be.

We take 450nm and 660nm as an example, from the SPD’s spectra, the reading of 660nm Y-axis coordinate is about 821.7, 450nm Y-axis coordinate reading is about 781.7, the difference between the two is about 5%. For the SQD spectrum, the difference is calculated as follows: 821.7 x 660 = 542,322 and 781.7 x 450 = 351,765, a difference of 54%. This is the reason why the difference between the SPD and SQD spectra is large for the height ratio of 660 nm and 450 nm.

### One Response 