September 8, 2013
September 8, 2013
I needed an easy to use small light to take photographs of small objects, so that I built it with white LEDs. Because it is intended to be used as a sub light source, it should be able to change the color tempearture. Of cource it can also be compensated by white balance feature of the camera. But sub light source should not differ with the ambient light and also sometimes be differed with the intention of lighting effect.
I used 10W white LEDs from Sharp Corp. for this project. It is also used in recent project, high power workbench light. The wattage of LED lamp may be considered too small for the photographic light but it has sufficient luminous flux for small objects from its high luminous efficiency and directivity. This also enables to use it at anywhere without AC power supply by using a small battery.
The intensity of black-body radiation and its spectral distribution is depends on the temperature of the body. The radiation of a black-body at high temperature is perceived by human eyes and it appears a color that varies depends on the temperature. The light color looks red, orange, yellow, white and blue-white as the temperature increases.
The temperature of body corresponding to the light color of radiation is called color temperature. Figure 1 shows the black-body radiation at different temperatures in the visible light. Note that the radiation intensity drastically varies depends on the temperature is normalized to the same power in the graph area.
To generate white light at arbitrary color temperature, everybody will design a tree color component light based on R-G-B LED. However the R-G-B LED is not used for generic lightings because of its poor CRI (color rendering index). The CRI is used to explain the ability of a light source to reproduce the colors of various objects compared to ideal light source, such as sunlight and incandescent lamp, as CRI is 100. For example of typical values, 15-50 for mercury-vapor lamp, 60-85 for fluorescent lamp and 90-95 for high CRI metal halide lamp. For photographic lightings, the CRI so high as 90 is needed.
Figure 2 shows the white light that a mixture of three color components, red, green and blue. As shown in the figure, the spectrum is not continuous. This means that the color in the spectrum hole cannot be reproduced. As the result, some colored objects will be appeared darken or different color. To achive high CRI by only emissons of LED, 5-7 colors will be needed to fill the spectrum hole. But on the other hand, narrow spectral light has high color purity, so that the R-G-B LED is ideal for display and light effects.
To change only the color temperature of the white light, it need not to produce all colors. It can be done by some white LEDs with different color temperature. The white LED for generic lighting is based on deep blue LED and phosphor. The phosphor is excited by the blue light and emits broad spectral of light as shown in Figure 3. This is an example of spectrums observed some light sources through a spectrometer. These color temperatures shown in the specsheets is defined by the light color corresponds to the black-body radiation and not that explaining the temperature of the light source.
Because also the white LED is based on luminescences, the spectral of emission is far off from black-body radiation and the CRI of the early white LEDs ware quite poor. However the CRI continues to improve and some type of white LEDs reache the CRI of 90.
In this project, two white LEDs, CRI=87, with different color tempearture are used. The color tempearture is controlled by current ratio of the LEDs. This is a kind of cocktail light but unlike generic cocktail light it is to change the color tempearture and not for improvement in the CRI.
The forward voltage of the white LED, 20V typ., is higher than supply voltage, so that L1 and Q1 makes a boost converter. The SMPS control is done by a microcontroller to eliminate DC-DC converter IC. Q2 is a dual MOS-FET to control the ratio of drive current of the two LEDs in complementary PWM. Because this is a photographic light need to be in steady power, the drive current is detected and controlled in closed-loop unlike open-loop control done in LED3 project. R1 is to limit peak current of inductor, R2 is to detect drive current for feed-back control.
White LEDs are mounted on a small heat sink found in the junk box. The size of heat sink with natural convection is insifficient to dissipate the heat of 10W LED, so that a cooling fan is attached to the heat sink to make a forced flow. LED chips are protected by an AR corted acrylic panel attached in front of the LEDs. The circuit board is mounted behind the heat sink.
The SMPS control is the main process driven by interprut. The two LED lights, 2700K and 6500K in color tempearture, is controlled with non-overwraping complemental PWM output of Timer 1. The mixing ratio of the two LED light is changed by WARM/COOL button and the setting is saved into the EEPROM.
The controller also moniters input voltage and output voltage. If low battery (<=8V) or output over voltage (>=25) is detected, it enters shutdown state.
