Mixed light scheme and simulation technology of supermarket fresh light

With the further development of solid-state lighting, people's requirements for lighting quality are getting higher and higher. Many lighting fields have proposed personalized spectral requirements. The lighting of fresh lights in food is a good example.

We know that under the lamp with low color and high color temperature, the face will look pale and lifeless. This is the same in food. With the inappropriate lighting, the fruits and food in the supermarket will become strange and unsatisfactory. Not only will it affect the business, but it will also be wasted because of the shelf life of the goods.

Under the appropriate fresh light, the color of the food can be better restored, making it more fresh and delicious, making customers more willing to buy, so the fresh light is very important in the field of food lighting. LEDs are an ideal source of light in applications due to their easy access to a wide range of spectral colors.

The presence of a variety of monochromatic optical chips and the use of special phosphors in the blue chip can make most of the spectrum realistic. But for a single LED, not every spectrum of LEDs will become a product, because the market demand for each particular spectrum of LED is generally not very large; even if there is a product, the price will be higher, and it is possible There is a supply issue. Relatively speaking, the color mixing scheme is more feasible. Since only conventional LEDs are needed, the light source cost is lower, the scheme is flexible, and the supply is more reliable.

We usually use color coordinates, CCT, CRI, and CQS to evaluate color. In addition to the color rendering index of the conventional 8 standard color samples, CRI also has a color rendering index of 7 special color samples, and these 15 color samples are relatively unsaturated color samples, the first 8 parameters The average value is our usual color rendering property Ra. For example, R9 characterizing red light ("strong red") is not within the characterization of Ra, so even if the Ra is large, as long as its spectrum does not have enough red light. In part, the value of R9 will also be low. NIST (National Institute of Standards and Technology) also found that even if a light has good color rendering properties for unsaturated colors, its color rendering properties for saturated colors may be poor; NIST found that only some saturated colors were selected. As a new color sample, it can guarantee the accurate characterization of color rendering, and propose a new method CQS (Color Quality Scale) which is used to characterize color rendering. It uses 15 kinds of distribution in the whole visible spectrum. The saturated color is its color. This article will use the two sets of methods to characterize color rendering to analyze the results after color mixing.

Given the spectrum, the various color information described above can be obtained, so the spectrum covers more comprehensive color information. For mixed-color lighting solutions, the spectral information is very important, but how to get it is more convenient?

First of all, although it can be obtained by measurement, it needs to make a sample lamp, which includes custom PCB, preparation of heat sink and purchase of power drive, welding and assembly steps. Finally, the measurement is complicated and time-consuming, and it is difficult to adjust the solution again. flexible.

Of course, the optimal spectrum required will vary with the subject being treated. Let's take the example of lighting a deep red apple. The fruit merchant hopes to get the right amount of deep red light doping, making the apple look rosy and delicious. At the same time, we must also emphasize that there are many kinds of red light. When selecting a red light source, we must first ensure that the correct dominant wavelength and spectral information are available, so that the mixed light scheme matches the lighting requirements.

In order to achieve our expected mixed-color spectrum results, we must first select the appropriate LED light source. OSRAM has a wide range of colorful products (including high-power OSLON series and medium-power Duris P5), especially in the case of red light, there are three kinds of products; white LEDs also have three color rendering indexes and various color temperatures. Therefore, this article takes its product as an example, selects the red LED of the appropriate dominant wavelength and the white LED of the appropriate spectrum to obtain their spectral information, and simulate it by LightTools software.

We use the (640nm) Red Red LED (LH) with the highest main wavelength value and the white LED (LCW) with a color rendering index of 80, using the (m LCW + n LH) color mixing scheme:

1. Obtain the spectral data of the LED. Normally, there are normalized spectra in the specification to obtain spectral data. If there is no spectrogram, the LED samples can be clamped by existing fixtures and easily obtained by the integrating sphere measurement system.

2. Enter the spectral data for each LED in LightTools. Simply copy the spectral data from the Excel spreadsheet into the software. And you can check the spectral input results through the "Spectral Region Chart", as shown in Figure 1.

Figure 1. Input and Spectrogram of Spectral Data for White and Deep Red LED Sources

3. Enter the radiant power/light flux of each LED in LightTools. In the specification, white LEDs are characterized by luminous flux (lm), while red LEDs are based on radiated power (mW). Here we do not have to convert the units and select the luminous flux directly in the software "Emittance" (" Photometric Flux”) or Radiometric Power, and enter the values ​​separately. Usually, when inputting the brightness information, it is necessary to consider the influence of the drive current and the junction temperature. However, as an example, the typical current drive is used, and the influence of the junction temperature and the like is ignored to simplify the process.

4. Run the simulation and analyze the evaluation of the spectral results. Of course, instead of getting the most suitable color mixing solution for the first time, you may need to go back to the third step, re-adjust the power/light flux ratio, and maybe even add or change the LED type.

5. Choose the appropriate color mixing scheme, and the corresponding color and brightness information.

Figure 2. CRI and CQS results for white LEDs

A separate analysis of the white LED spectrum reveals that although Ra = 83, the R9 value representing the red component is only 11, indicating that the source has poor color reproduction for this red object and is not suitable for highlighting the illumination of red objects. This is why the red light of the red food needs to be enriched with a certain amount of red light, which also highlights the importance of the fresh light. Of course, it is not possible to add red light excessively. Otherwise, although the red reducing ability is high, the color rendering of other colors is greatly reduced.

Assuming that the ratio of the white LED to the red LED is k=M:N, then the number of white LEDs should be set to k times the white light flux. We assume that each red LED is 0.3W radiated power, and each white LED is 100lm. If you input 500lm white light, then k=5.

Figure 3. Mixed light spectrum and its CRI and CQS results

Add "Far field receiver" and run the simulation. It was found that this scheme well added the red light spectrum, CCT=2750K, and got better CRI(92) and CQS(90), in which the index of red reduction ability is R9=70, VS1=90, The main reason for the difference is that the selection of the red standard color samples is different, and the results are higher than the white LEDs, indicating that this fresh light scheme is more conducive to the color reduction of red foods; The index also retains higher or higher values. While achieving the red-reducing ability of the fresh lamp, the overall lighting quality is also improved.

Conclusion: The requirements for the spectrum of lamps in food lighting and other fields are more and more diversified, but the source of custom spectrum is difficult to mass-produce, and it is expensive, so the color mixing scheme is usually adopted. Due to the existence of the viewing curve and the spectral curve of the light source, the calculation between the radiant power and the luminous flux is very troublesome. In this paper, the light mixing spectrum is simulated by a simple setup using the existing tool LightTools, except that the spectrum is obtained. It can also get a lot of information about other colors; the results show that the lighting effect obtained by this program is very good, the color rendering is more than 90, rich in deep red, suitable for lighting many red foods.