MAX44007 ambient light sensor improves black glass design performance

Black glass changes the spectrum of light that hits the ambient light sensor, which is a design challenge for ambient light sensors. In particular, black glass enhances the infrared portion of the spectrum, which is invisible to the human eye. This application note describes several calibration/compensation methods to correct the lumen readings of the light sensor under different lighting conditions. This article discusses how to use the advanced modes of the MAX44007 light sensor to adjust its response to visible light and IR channels. The MAX44007 registers can be used to optimize the sensor's performance under black glass.

introduction

The MAX44007 ambient light sensor offers an advanced operating mode to optimize the sensor's performance behind black glass.

Currently, most smartphones, tablets, laptops and TVs use black glass as the frame of the LCD screen, which makes the terminal product look exquisite and beautiful. In the traditional design, a clear circular window or gap is provided at the location of the ambient light sensor. In the new product, the window or gap of the light sensor is covered with almost opaque black ink, which blends in with the surrounding color. The reason for the manufacturer is simple: the darker the color, the less likely it is to distract the user, and it will look more professional and more sophisticated!

Unfortunately, dark ink covering the ambient light sensor complicates the design in two ways. First, the dark ink attenuates the ambient light, thus reducing the amount of light received by the sensor. Second, the ink also changes the spectrum of the light. The spectral characteristics of the ink almost entirely pass the incident infrared light, while the visible light is attenuated to 3% to 5% of its original intensity. Therefore, the infrared content of the ambient light is greatly amplified. The actual chemical properties of the dark ink will vary from supplier to supplier, further complicating the characteristics of the optical signal transmitted (infrared or ambient light).

Calibrated and compensated for black glass

It is very difficult to accurately match the CIE curve of the human eye. For this reason, most high-performance ambient light sensors (such as the MAX9635) have a calibration/compensation mechanism to correct the lumen reading under different light sources. This correction is achieved by combining two on-chip photodiodes, which effectively provide an accurate light intensity reading to eliminate the effects of different lighting conditions. Since the light spectrum shining on the sensor under black glass changes significantly, further adjustment of the calibration parameters is required.

Special attention needs to be paid to the effect of the light source on the sensor under the black glass and the calibration needs to be corrected. Correction is especially necessary if the light source has a high infrared component, such as sunlight and incandescent light. White LED (WLED) and fluorescent lighting require less adjustment.

The MAX44007 features advanced modes that can be used to adjust the response to visible light and IR signals.

One-time presetting, power-on setting

Follow these steps to perform a one-time preset, usually set at power-up.

Read the contents of each of the 4 registers: 0x09–0x0C.

Storage variables: Adv1, Adv2, VisibleGain and IRGain respectively.

Save the 1's complement of these variables into new variables: Adv1C, Adv2C, VisibleGainC, and IRGainC. For example, IRGainC = !IRGain;

If IRGain = 1010 0110, then IRGainC = 0101 1001.

Write 1000 0001 to register 0x0D to enter advanced mode (set ADV to 1).

Write Adv1C, Adv2C, VisibleGainC, and IRGainC to registers 0x09–0x0C, respectively.

For example, writing 0101 1001 to register 0x0C has an original value of 1010 0110.

Note: A subsequent read of register 0x0C (after the above write operation) will still read back 0101 1001 because the IC does an internal automatic bit flipping before saving the data into these advanced registers.

The IRGainC value is reserved for future use.

If necessary, set an appropriate delay for the Gate Timing Register 0x07.

Set INTE = 1 (register 0x01) to enable interrupts.

Enter normal working mode

Follow the steps below to enter normal measurement mode.

Read registers 0x03 and 0x04 to get a 12-bit lumens reading.

Save the value as ComboLux.

Write 0000 0000 to register 0x0C to enter temporary measurement mode.

Wait at least 1.6s (2x 800ms).

If necessary, contact Maxim to shorten this time.

Read registers 0x03 and 0x04 to get a 12-bit lumens reading.

Save the value as ApproxLux.

Write IRGainC to register 0x0C to exit temporary measurement mode.

Calculation: ActualLux =ApproxLux - IRFactor × (ApproxLux - ComboLux)

AcutalLux is the actual ambient light reading.

ApproxLux = ComboLux, approximating fluorescent and WLED lamps.

ApproxLux > ComboLux, usually incandescent light and sunlight.

To calculate the appropriate IRFactor, please contact Maxim Applications Support and provide a glass sample. Data can be obtained directly from laboratory testing.

According to the ActualLux calculated above, set the corresponding backlight intensity.

Using ComboLux as a reference (normal operating mode of the device), set the Lumen High Limit (Register 0x05) and Lumen Low Limit (Register 0x06) accordingly.

Perform a dummy read of the INTS bit (register 0x00) to clear all interrupts if they were previously set.

Waiting for hardware interrupt.

This is when the program takes the longest time.

After a hardware interrupt occurs, read register 0x00 and verify that INTS = 1.

If INTS = 1, go to step 7 above.

Otherwise, if INTS = 0, return to step 16 after checking other hardware interrupt sources.