New integrating sphere and spectrometer

Integrating sphere (aka spherical integrator) with a spectrometer is a laboratory device for measuring primarily the luminous flux of a light source. The measured values are highly accurate thanks to the calibration of the entire device. We will use it mainly to confirm that the values we give, primarily lumens, are real. At the same time, it will also serve us for further development, such as for the development of new optics, when we will be able to accurately measure the differences between different TIR optics and select the best one, or develop and verify it.

Why is luminous flux measurement so complicated?

Of all the various (electrical) quantities, luminous flux is measured the worst and generally with the least accuracy. While you can quite easily measure voltage and current with an accuracy of millivolts and milliamperes or less, measuring luminous flux in laboratory conditions is "only" with an accuracy of +-2% to +-5%. This is because there are many inaccuracies in the measurement (reflectivity of the inside of the sphere, time since calibration, noise in the device, parasitic light) including the fact that the human eye perceives different wavelengths to different degrees, so the measurement must be converted to photometric values (lumens) and any inaccuracy in the spectrum affects the result.

Accurate measurement of luminous flux is therefore very demanding and requires such expensive laboratory equipment, common consumer/consumer devices have measurement inaccuracies of up to 20% and are unusable. Accurate measurement of luminous flux can be done either with a goniometer or in a spherical integrator. There are several manufacturers of similar devices, usually at least one on each continent, after an internal selection we finally chose the 50cm spherical integrator from the German company Gigahertz-Optik due to the greater versatility of the device, ease of delivery and future service.

Technology and measurement procedure

The measuring device (spectrometer) usually works in such a way that the light beam passes through a diffraction grating and is divided into individual wavelengths, which fall on the detector array. The detectors are in principle uncalibrated with different response to the signal, so they must always be calibrated. The BiTec sensor in the BTS256-LED measuring device contains an array of detectors and also a photodiode, which helps to make the spectrometer more accurate, because the photodiode is highly linear and stable.

The spectrometer has a USB output to a computer and its own measurement software, where all measurements are made. The entire measurement process is longer and it is necessary to follow the given steps. At the beginning, we turn on the calibration lamp, calculate the correction and then perform the actual measurement - this usually takes about 1-3 seconds. The software can display both spectrometric values (spectrum), photometric (luminous flux) and colorimetric (CCT, duv, CRI and other indices).

Parameters

• Optical resolution: 5nm
• Data resolution: 1nm
• Max. measurement range (50cm sphere): 1.2-120000lm
• Calibration: ± 5% for luminous flux