LED Color Measurement with Compact Spectral Sensor

The Spark spectral sensor is easily embedded into handheld colorimetric systems for applications including color measurement of studio lighting, large projection displays and LEDs in sorting operations. In this application note, we test Spark feasibility by measuring a multi-color LED strip.

About the Spark Spectral Sensor

Spark is a spectral sensor that fills the price-performance niche between diode filter-based devices and spectrometers. Spark is a visible sensor that adds a new dimension both in size – which is about the same as a small microcontroller — and in cost reduction, but with the ability to perform full spectral analysis from 380-700 nm (Figure 1).

Unlike traditional spectrometers, where a finite volume is required to fold and split the incident beam of light from the sample, Spark uses a form of solid-state optical sensing to reduce the light from the sample into its component wavelengths. For all intents, the Spark is a detector chip that functions as a spectrometer, producing a digitized spectrum from a 1024-pixel linear silicon CMOS array.

Because of its size and affordability, Spark is an especially attractive option for embedding into OEM devices and integrating onto process lines. It can be used for all types of spectral measurements including absorbance, color, emission and fluorescence.

Emissive Measurements of LEDs and Other Lighting

Spark can be used successfully for characterizing lamps and illuminants in the visible range, including LEDs and other sources. Indeed, Spark makes an ideal testing tool for LED production and for the implementation of LEDs in specialized lighting applications, where it can be used as a handheld spectral light meter for photographic and theatrical lighting measurements. Unlike photodiode sensors, Spark is a full spectral range device, which makes it possible to measure lighting characteristics such as correlated color temperature and color rendering index.
 

The sampling setup does require some finesse when using Spark. Because of the Spark’s optical design, even intensity of illumination across its aperture is recommended. This can be accomplished by adding to the Spark a diffuser accessory (Figure 2), which is recommended for free space measurements including emissive measurements from point sources.
 

For some relative measurements, even intensity of illumination across the Spark aperture is not necessary, as any variation in intensity across the aperture may be referenced out. However, for absolute measurements, it is essential that the lighting and geometry of the Spark setup is identical when calibrating the device and when taking a measurement. Any variation from the calibration setup (moving of fibers, light source or device, or altering the background lighting) will nullify the calibration.

Experimental Conditions
 

To measure the emissive color of a multi-color LED light strip, we used a Spark spectral sensor (380-700 nm) with diffuser accessory (Figure 2). Spectra were measured in relative irradiance mode using OceanView spectroscopy software. Relative irradiance measurements are used when only the shape of the emissive sample is needed. (Absolute irradiance measurements with Spark using a free space setup would require calibration at our facility using predefined front-end optics.)

Relative irradiance calibration was performed using an HL-2000-HP high power tungsten halogen light source. Regardless of the emissive source being measured, intensity calibration is required. This applies even if the goal is to simply make color measurements of an emissive source like an LED, computer display or light source. If the sample under study is emissive, you must begin your measurements by calibrating.

Results

With integration time set to 59 ms and averaging set to 25 scans, Spark measured the spectra of red, green, blue and white LEDs

 

LED color spectra were measured using the Spark spectral sensor.

As shown in OceanView, results can be plotted on the CIE chromaticity diagram and calculated as color values including dominant wavelength, hue and saturation

 

With OceanView software, Spark users can capture color spectra and calculate color values

With Spark, similar LED color measurements can be scaled for large-volume LED production and sorting operations, or integrated into systems for OEM applications. Whether used alone or embedded into a measurement system, Spark is ideal for color and other applications requiring a low-cost, small-footprint visible spectral sensor.