An ideal quantum sensor would give equal emphasis to all photons between 400 and 700 nm and would exclude photons above and below these wavelengths. The response of such a sensor is shown in the adjacent graph below. The most accurate way to measure this radiation is with our spectroradiometer, which costs less than $4000. Our quantum meters are accurate to within about ±3 % for common light sources.

The spectral response of the Apogee Sensor used in Quantum Meters and the Quantum Sensor is shown at right. As the figure indicates, the sensor underestimates the 400 to 500 nm wavelengths (blue light), overestimates the 550-650 wavelengths (yellow and orange light), and has little sensitivity above 650 nm (red light). Fortunately, common light sources are mixtures of colors and the spectral errors offset each other. The sensor measures green light (500-550 nm) accurately, so it can be used to measure the radiation inside and at the bottom of plant canopies.