A Single Photon Avalanche Diode (SPAD) sensor helps investigate color-to-color LED flicker by providing extremely high-speed, time-resolved, and single-photon-sensitive measurements. When combined with a color filter array, a SPAD array can precisely detect rapid fluctuations in the light intensity of different color channels, revealing the dynamic color shifts of the LED over time.
Exceptional time resolution: SPADs can time-stamp the arrival of individual photons with picosecond precision. This unparalleled temporal resolution is crucial for detecting very fast light intensity variations that can cause color flicker. Standard cameras often lack the frame rate needed to capture these rapid changes.
Single-photon sensitivity: SPADs are optimized for detecting low-light signals down to the single-photon level. This high sensitivity allows them to detect subtle fluctuations in light intensity that conventional sensors might miss, which is especially important for analyzing low-intensity flicker or variations in the dimmer regions of an LED's operating cycle.
High dynamic range and noise resistance: Unlike conventional Charge-Coupled Device (CCD) or Complementary Metal-Oxide-Semiconductor (CMOS) sensors that measure the accumulated amount of light, SPADs count individual photons. This digital photon-counting approach prevents the analog noise that can contaminate data in low-light conditions and allows for a very high dynamic range.
Color-filtered arrays: To investigate color-to-color flicker, SPADs are arranged in an array and covered with a color filter array (CFA), such as an RGBW (red, green, blue, white) pattern. The sensor can then separately measure the photon counts for each color channel. By analyzing the photon-timing data for the red, green, and blue pixels, researchers can reconstruct the color variations and precisely track how the light output of each color channel changes relative to the others.
Gated imaging: Some SPAD cameras offer "gated imaging," where the sensor is only active for a very short, precisely timed exposure (down to nanoseconds). By taking a series of these gated images with a slightly shifted time delay, the system can reconstruct a high-speed video of the light output. This is a powerful technique for visualizing and analyzing how the timing and intensity of different colors change within a single flicker cycle.
How SPADs reveal color-to-color flicker
To detect color-to-color flicker, a SPAD array is configured to measure the LED light source. The process would typically involve these steps:
High-speed photon counting: The SPAD array, with its color filter, counts the number of photons hitting each pixel over very short time intervals (e.g., nanoseconds).
Separate color channels: The color filter ensures that only photons from a specific wavelength range reach certain SPADs. The sensor effectively records separate, high-speed intensity curves for the red, green, and blue light components.
Data correlation and analysis: The time-stamped photon data for each color channel is then analyzed. Researchers can plot the intensity curves for each color and observe how they rise and fall relative to one another.
Identifying discrepancies: By comparing the high-resolution timing of the different color channels, researchers can identify subtle discrepancies in their light output. If the red and green channels turn on or off at slightly different times, or if their intensities fluctuate out of sync, this indicates a color-to-color flicker.
The combination of single-photon sensitivity, high timing resolution, and color-filtered arrays makes SPAD sensors an ideal tool for revealing the rapid and subtle spectral variations that are otherwise invisible to the human eye or slower measurement techniques.