Embedded microcontroller technology allows the colour components of a light source to be sensed and actively controlled. Active colour control can be used to create a specific colour of light or can be used to create a mix. For example, multiple component colours can be blended to create a specific quality of white light.
Applications for active colour sensing and control include:
• Manufacturing and quality control
• Medical instrumentation
• LCD video panel backlighting
• Film, video and photography
• General architectural lighting
There are different ways to quantify the quality of a white light source. The Correlated Colour Temperature (CCT) of a light source compares the perceived colour to that of a black body radiator at a certain temperature. White light sources with a low CCT emit a more reddish quality of light, while those with a high CCT emit a bluish quality of light. The Colour Rendering Index (CRI) of a light source specifies its ability to reproduce a broad range of colours. A monochromatic light source has a very low CRI value, while an incandescent light bulb would have a relatively high CRI value.
Colour Sensing
A variety of colour sensors are available that are compatible with embedded systems. These sensors typically generate analog output voltages or a variable frequency digital output. The sensor will provide an output for red, green, and blue colour components. A PIC® MCU with an ADC can be used to read the analog colour sensor. For the digital output sensor, a digital timer can be used to count pulses over a certain amount of time. Or, an input capture peripheral can be used to measure the period of the sensor output signal.
Output Control
Pulse Width Modulation (PWM) signals provide an effective method to control the colour of light produced by a system, regardless of the lighting technology that is used. The PWM signals can directly modulate the light source to control the average on time, or they can be filtered to produce analog control voltages. Either way, the ability to produce high resolution PWM signals is often required. High control resolution allows accurate colour control over a broad range of brightness levels. Three PWM peripherals are typically required in a colour control system for the control of Red, Green, and Blue colour information.
Recommended Products for Colour Control
Low-Cost 8 Bit
When extremely accurate colour control is not required,
software PWM algorithms can be used to generate multiple control signals with very low cost devices.
The
PIC16F7x7 family of MCUs provides a low-cost 8-bit solution for active colour control. These devices have three PWM channels with up to 10 bits of control resolution.
High-Performance 8 Bit
The
PIC18F1230/1330 devices provide three 14-bit PWM channels for high resolution control. The CPU of the
PIC18F device family devices has additional math resources and operates at a higher speed to allow faster execution of control loops.
Low-Cost 16 Bit
The
PIC24F family of
16-bit MCUs offers several benefits for colour control applications, including:
• Up to 5 Output Compare modules that can generate 16-bit resolution PWM signals
• 16-bit CPU for fast algorithm execution
• 10-bit ADC
• Multiple communication peripheral options
High-Performance 16 Bit
Devices in the
PIC24H family offer up to 8 output compare peripherals that can generate 16-bit PWM control signals. In addition, this family of devices has options with a 12-bit ADC for high resolution colour sensing.
