Calibrating the White LED Lights of an Autolab Spectrophotometer

This article is focused on the correct procedure to be followed to determine the value of responsivity while calibrating white LED lights on the Metrohm Autolab Optical Bench. This has three types of white lights as presented in Table 1, as well as colored lights.

Table 1. List of the different white LED lights available for the Metrohm Autolab Optical Bench.

The white lights are named for the perception of the color and not in relation to the temperature, because higher temperatures cause light emission which appears cool because of the blueish hues, while lower temperatures cause emission of warm white light with a more orange tinge. Neutral white, on the other hand, has little of either.

The light intensity of the white LED lights changes with duration of use, and periodic recalibration is recommended for this reason.

In this test procedure, cool white LED light is used but the principles may be utilized in all types of white LED lighting.

The Metrohm Autolab Optical Bench contains a calibrated photodiode which is useful in this procedure. Each photodiode that comes with the bench has its own associated chart plotting responsivity values η (in A/W) against wavelength λ (in nm). Depending on the LED light color that is selected for calibration, the appropriate responsivity is found, such as 0.32 A/W for a light with a wavelength of 627 nm (a red LED light).

White light presents a difficulty in that it is not comprised by a single color but a mixture of several wavelengths. Thus in this case the responsivity values taken for a preset array of wavelengths are multiplied by normalized intensity values.

In other words, the intensity of light at each wavelength decides the percentage of the responsivity value to be included in the calculation. The light intensity from the white LED light is collected between the wavelengths of 400 nm and 700 nm and normalized for this purpose, to be used as weighting factors in calibration.

Experimental Setup

A Metrohm Autolab PGSTAT204 is used along with a Metrohm Autolab Optical Bench and any of the white LED lights.

Figure 1. On the left, the rail equipped with an LED light and the photodiode. In the center, the Metrohm Autolab PGSTAT204. On the right, the LED driver.

An Autolab spectrophotometer (Figure 2) will help to tabulate the light intensity between the extreme ends of the visible light spectrum, namely, 400 nm to 700 nm. This is connected with an optical fiber of 200 µm diameter which is positioned 20 cm from the light source.

Figure 2. The Autolab spectrophotometer.

The surroundings must be kept darkened to exclude the artefactual effects of external light from the measurements being taken.

The Procedure

The spectrometer collects readings of light intensity and these are plotted against the wavelength, and then normalized to the maximum value obtained (Figure 3).

Figure 3. Normalized intensity vs. wavelength of a cool white LED light

The photodiode chart showing responsivity values for each wavelength, that is provided by the manufacturer, is now consulted to select the 400 nm to 700 nm range of wavelengths that corresponds to visible light, to fit the readings taken from the white LED light. (Figure 4)

Figure 4. Responsitivity vs. wavelength plot, specific to the photodiode used for the measurement.

Since white LED lights are limited to the suitable wavelength ranges of visible light, 400-700 nm, Figure 5 shows a truncated version of the data in Figure 4 to match this.

Figure 5. Responsitivity vs. wavelength plot, from 400 nm to 700 nm.

A more delicate step is to correctly apply weighting factors from the just derived normalized intensity values to the responsivity values. This requires that the wavelengths of both be matched correctly. This in turn means that both responsivity and intensity be at the same wavelength, for instance, at 400 nm. To obtain the normalized light intensity values for the selected wavelengths from the responsivity vs wavelength plot, the interpolate command on the NOVA software is applied. Once this is done, the average of the weighted responsivity values over the whole visible spectrum can be found using the following equation, where w are the weights and λ represent the wavelengths:

In this case the value obtained is and this is used to calibrate the white LED light.

Learn more about calibrating white LED lights of an Autolab Spectrophotometer

Conclusions

This sets forth the procedure to be used to get the correct value for photodiode responsivity, which is essential to calibrate the white LED lights. First the white light intensity must be collected via a spectrometer and normalized, and then the responsivity vs wavelength values found from the plot accompanying the photodiode. These values must be weighted by the normalized intensity values and finally the average of the weighted responsivity value so obtained, is found.

This information has been sourced, reviewed and adapted from materials provided by Metrohm AG.

For more information on this source, please visit Metrohm AG.