Spectroscopy measurement with Christmas light bulbs

This article describes how to use Ocean insight's high-speed Ocean FX spectrometer to determine the flicker, color, and other spectral characteristics of incandescent and LED light bulbs.

Everywhere you go during the holiday season, you'll find that the surroundings are filled with enchanting lights. The colorful and twinkling lights also give a pleasant atmosphere. So this time, we are going to learn about the spectroscopic characteristics of Christmas lights.

LEDs are widely used for lighting because of their low power requirements, high efficiency, and bright colors. But here you may be wondering what the difference between an LED and an incandescent bulb, which serves the same purpose. Not everyone will choose a light based solely on efficiency. The following describes my findings while checking the irradiance, flicker, and color of different colored LEDs and incandescent bulbs used for lighting.

※ Experiment method

Ocean-FX-VIS-NIR Spectrometer (350-1000nm)

FOIS-1 integrating sphere

600µm Vis-NIR fiber

OceanView software

Twenty multi-color incandescent and LED lights were tested using the above product (Figure 1). The Ocean FX can do up to 4,500 scans per second, making it perfect for measuring fast flicker in light. Spectral data were measured for three bulbs of each color in absolute spectral radiant flux (uW/nm) using a radiometrically calibrated HL-3P-INT-CAL light source as a reference.

OceanView's 'processed mode' was used to correct the Ocean FX spectrometer spectral data to give intensities in microwatts per nanometer (uW/nm).

Figure 1. Ocean FX Spectrometer

As mentioned above, we experimented with the Ocean FX because its fast scan rate allows it to measure high-intensity LED light without saturating the detector. Additionally, because of the high scan rate of the spectrometer, it is possible to qualitatively evaluate the flickering of the blue bulbs from each LED string (change in brightness over time related to output variation). Quantitative color measurements were also taken for each color of the LED and incandescent bulb strings.

※  Experimental results: Spectrum data of incandescent bulbs vs. LED Christmas lights

Incandescent light bulbs used for Christmas lighting show a broad spectrum with the strongest intensity in the short wavelength region of the NIR >700 nm (Figure 2). The bulb warms up during operation because light in that wavelength range generates heat beyond the range that the human eye can see. In fact, according to the US Department of Energy, 90% of the energy emitted by an incandescent light bulb is heat.

Figure 2. Spectrum of an incandescent light bulb; Heat is generated in the wavelength region of 700 nm or higher with the highest intensity

Also, the spectrum of an incandescent bulb looks similar to the spectrum of a high pass optical cutoff filter. These spectral shapes come from the glass cover that gives the incandescent bulb its beautiful color. The glass cover acts as an optical filter that transmits the desired wavelength.

The LED light string has no signal in the short wavelength region of the NIR >700 nm and has a narrow emission spectrum, so the bulb stays cool even after long hours of operation (Figure 3). In addition, LEDs not only generate less heat than incandescent bulbs, but are also more energy efficient. When the absolute spectral radiant flux was compared to the power of an incandescent bulb, LEDs showed a much higher intensity. The reason for this difference in signal strength is that the LED bulb generates the color directly from the bulb, rather than using a filter to select the desired wavelength from the light-emitting filament.

Figure 3. Spectrum of LED; Shows no signal in the wavelength region above 700 nm

※  Experiment result: flickering characteristics of Christmas lights

Lighting powered by alternating current (AC) may exhibit intensity fluctuations depending on AC power frequency. Fluorescent lights are one type of light that fluctuates a lot, which can fluctuate in intensity by up to 20% (which is why some people prefer natural light). The human eye is less sensitive to intensity fluctuations at frequencies below 70 Hz, but at frequencies up to 200 Hz, some people may experience eye strain and headaches, and in severe cases may lead to blurred vision. Besides this, flickering light can also cause other health effects, such as seizures and nausea.

One of the difficulties encountered in analyzing the blinking of light is the lack of instruments capable of characterizing a light source on the time scale of blinking (often faster than the human eye can perceive). But with the Ocean FX, you can measure the entire band (350-1000 nm in this experiment) at 4,500 scans per second, so you can see flicker imperceptible to the human eye.

Flickering could be seen for incandescent bulbs and blue bulbs in LED lights. (Unlike some decorative lights, the lights we used in our experiment were not designed to flicker). Although no flicker was visually observed, the spectra in Figures 4 and 5 show a dramatic difference between incandescent and LED bulbs. For example, if you look at the following y-axis ranges from each other in each plot, the measured intensity change for the light bulb is significantly different. Blue incandescent bulbs show fluctuations of 1-1.75 uW/nm, LEDs 0-700 uW/nm.

Figure 4. Blue incandescent bulbs have low flicker characteristics, with little change in intensity over time.

Figure 5. Although no intensity fluctuations are visible, the spectral output of the blue LED bulb shows  a higher flicker than the blue incandescent bulb.

Furthermore, oscillations of consistent intensity are observed in LEDs, whereas incandescent bulbs exhibit random fluctuations. These differences have to do with the nature of the bulb itself and not with differences in settings. The filaments used in incandescent bulbs are not as sensitive to current changes as LEDs. Both measurements were made with identical settings and parameters while connected to the same AC power source. Although this experiment provides a qualitative assessment of blinking, a similar type of high-speed measurement can also be used to provide a quantitative assessment of blinking.

※  Experiment result: Color measurement of Christmas lights

The beauty of Christmas lights comes in the different colors of the lights. Our eyes see lights as green, blue, orange, yellow, etc., but quantitative spectroscopic measurements are needed to determine the exact color of the light.

We offer several spectrometers useful for characterizing a variety of roughness parameters, including color. Quantitative color values ​​for incandescent and LED bulbs are shown in Table 1. These values ​​are matched to a location on the CIE chromaticity diagram to define a color as a weighted sum of the three primary colors. To simplify the complex system of classifying colors using the CIE color space, blue is represented by low x, y values, green by high y values, and red and orange by high x values. Quantitative color is a much more accurate measurement of color, as evidenced by the large difference in values ​​for the same color bulb.

Table 1. Quantitative color measurement for incandescent and LED Christmas lights

※ Conclusion

A compact system consisting of several components was used to measure various parameters of Christmas lights, including characteristics imperceptible to the human eye. OceanInsight spectrometers and accessories can be used to measure various properties of light.