Talk about some index concepts of LCD projector

2022-08-01
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Talking about some indicators and concepts of LCD projectors

as an engineer and university teacher specializing in the research and development of projection display, the author saw some recent introductions on optical knowledge and devices of projectors in some electronic newspapers and periodicals, and found that there were serious conceptual errors, so he wrote this article to correct the audio-visual situation

luminous flux

many manufacturers and consumers often refer to the "luminous flux" of projectors as "Brightness". In fact, in terms of optical concepts and definitions, brightness and luminous flux are two completely different concepts. According to the international system of units (SI) of photometry, brightness refers to the luminous intensity of the panel at a point on the surface in a given direction divided by the orthographic projection area of the panel on the plane perpendicular to the given direction. Its unit should be candela per square meter. The brightness of a front projector is different in different directions (angles) and distances. Therefore, it cannot be used to measure the quality of the projector, nor can it be used as a parameter of the projector. Only display devices with fixed screen geometry such as CRT display and rear projection may adopt the concept of brightness. Luminous flux is the amount of light emitted by a light source in a unit time. It is a measure of the radiant power of that part of the light source that can be perceived by the human visual system. Its unit is lumen or ANSI lumen. Under standard measurement conditions, the luminous flux of a projector is generally basically constant and is the main parameter of the projector. In the measurement of luminous flux, the core is to correctly measure the average illumination of the screen and the effective area of the screen. We have tasted the benefits of win-win cooperation. The product is the luminous flux of the projector. In actual operation, it is difficult to measure the illumination. This is because it is difficult for an ordinary illuminometer to accurately measure the actual illumination of different color temperatures. Only an illumination probe that accurately compensates for each color temperature is often worth more than 10000 yuan

in the original ANSI standard describing luminous flux test, the luminous flux test is defined as "a projector is placed 2.4 meters away from the projection screen to project a 60 Inch full white picture...", but imagine: can all projectors project a 60 inch picture at a distance of 2.4 meters? The answer is No. due to the differences in optical structure and size of projectors of different brands and types, the projection size/projection distance are different. This method can not be copied in the actual measurement. The author shows through a large number of actual measurements that the error of measuring the luminous flux of the same projector under different projection distances (i.e. different projection screen sizes of thermoplastic molding and extrusion materials) is not more than 5%, which is basically the same. In addition, in the measurement, attention should be paid to the environmental illumination, power supply voltage, test picture (standard white field) quality, projector parameter settings (such as brightness, contrast, color, etc.), illuminance meter test accuracy, and the impact on the measurement results. Especially in the multi machine comparison test, attention should be paid to the scientificity and consistency of the test conditions. After a long-term test of the luminous flux of various projectors, the author found that the luminous flux index of famous projectors in Japan, the United States, South Korea and other countries is generally 5% ~ 30% moisture, and that of domestic projectors is generally 15% ~ 40%. The index error of a certain model of XXXX electronic research Institute is as high as 73%, which is of little practical significance

luminous flux is the main parameter of the projector. How to obtain high luminous flux when designing and making the projector? Some domestic second rate manufacturers arbitrarily believe that the main way to increase "Brightness" (i.e. light flux) is to increase the bulb power and the total light flux of the bulb. In fact, this is the most original method. The majority of fans denied this method three years ago. The main methods to improve the optical quality (luminous flux, optical uniformity) of the projector should be to design a scientific and rigorous optical path (optical engine), improve the efficiency of the optical path, and try to use professional and efficient optical devices at a reasonable cost. For example, in the main low-end models of a company, in order to reduce the LCD screen temperature unilaterally, the reflective bowl and reflective cup were abandoned. The utilization rate of this light source alone decreased by 40% ~ 60%. Of course, the "Brightness" of this machine is very low. In addition, the focusing lens, plane mirror, liquid crystal slice, 2-3 Fresnel mirrors, projection lens and other optical devices commonly used in domestic low-end single-chip liquid crystal projectors have an impact of 8% ~ 30% on the total luminous flux, and the design size of the optical path has an impact of about 8% ~ 25%. Based on the above factors, the light path design and the material of optical components will have a 40% ~ 200% impact on the total light flux of the projector. Therefore, in the author's actual measurement, it is found that the "Brightness" difference of projectors of different brands is more than double when using the same 150W OSRAM hqi-150d bulb. Generally speaking, 80ansi ~ 1000ansi lumen projectors are required for video applications within a 100 square meter field; Audio visual education and business need 400ansi ~ 3000ansi lumen projectors; Large scale engineering and outdoor applications require high brightness professional projectors above 3000ansi lumens

color temperature

for color temperature, we should first clarify the concept of color temperature and the measurement of color temperature, and then talk about the selection of color temperature

for the color temperature of different light sources, there are several concepts: distribution temperature, color temperature and related color temperature. At present, the bulbs used in all types of LCD and DLP commercial projectors are gas discharge lamps. The concept of common "color temperature" is not suitable for projection lamps. This is because the gas discharge lamp for projection purposes is different from the ordinary tungsten filament lamp, and its spectral power distribution is irregular. The concept of "correlation TEM" shall be adopted in terms of color temperature, which is defined as: if the chromaticity coordinates determined by the relative power distribution of the radiation source (mainly for the gas discharge lamp) at temperature T are not on the blackbody temperature track of the chromaticity map, Near this trajectory, the temperature of the radiation source is represented by the chromaticity coordinates and the nearest blackbody temperature, that is, the relevant color temperature of the radiation source

according to the author's understanding, at present, only a few domestic projector manufacturers, such as Ruicheng technology, can complete the measurement of the color temperature of projection bulbs and pictures. Most manufacturers do not have the minimum instruments to measure the color temperature at all, and even do not know the measurement method of the color temperature at all. Completely rely on the eyes to "measure" the color temperature, and then talk about the color temperature parameters and concepts to the market and the media, which makes the author afraid and the consumers thrilled. Generally speaking, the color temperature of projector bulb (gas discharge lamp) and picture shall be measured by "spectral power distribution method" (the measurement error is less than 50K). The "two-color method" or simply "two-color" and "three-color" color thermometers (the measurement error may be as high as 1000K) cannot be used. From a large number of actual measurements, the author found that for bulbs with strict quality, such as OSRAM and Philips, the color temperature dispersion of batch products is still as high as about 600k, so the color temperature parameters in the bulb specification are not completely reliable

as for the range of color temperature which is more suitable for human vision, it is a matter of different opinions. The color temperature of most high-end projectors and monitors is adjustable, generally ranging from 6000K to 10000K (as can be seen from the specifications of many projectors and monitors). According to a senior academician of the Chinese Academy of Sciences who is engaged in the research and development of projectors, the general eye tends to favor the color temperature of 6500k to 9500k. At present, the standard color temperature of Japanese radio and television is also 9300k. According to the measured color temperature of a large number of famous brands LCD The color temperature of DLP projector is generally 6500k ~ 10500k. As for whether the high-color temperature projection bulb will burn the skin and damage the eyesight as some lovers say, it depends on how you use the projection lamp, just like a kitchen knife can cut vegetables and kill people. Moreover, high-quality small double ended metal halide lamps such as BLV, OSRAM, Philips and Ge have reliable UV protection and fully comply with CE, UL and other safety certifications. However, a user who has no knowledge of Optics and electricity may still be damaged due to improper use. Even a 200W ordinary incandescent bulb may cause harm to human body in a very close distance, but this has nothing to do with the quality of the bulb

contrast

contrast is also a very important indicator of projectors (especially for video projectors). Projectors with high contrast often make people feel that the picture is "clean" and "stable". But there are many ways to measure contrast. Contrast itself means the ratio between the maximum illuminance of the picture divided by the minimum illuminance. For the same projector, you can divide the maximum illuminance of the pure white field by the minimum illuminance of the pure black field, which will be very high. You can also divide the average illuminance of the "white area" of the black-and-white picture by the average illuminance of the "black area", which will be low. The indicators of contrast are confused among various projectors and brands. Now many LCD projectors in Japan and South Korea have been "marked" above 600:1, but their visual quality may not be as good as the 300:1 European and American models. This is the artificial difference caused by different measurement standards and commercial means. At present, there are some terms on the contrast index, such as maximum contrast, typical contrast, American standard contrast, Japanese standard contrast, etc. However, in the case of no major breakthrough in liquid crystal material technology, contrast will not be the advantage of liquid crystal displays and projectors

comprehensive factors of projectors

the reason why projectors are called "projection" is that they use a large number of optical devices for projection purposes. The technical core and cost ratio of optical devices exceed the weight of circuits. It can be seen that the projector is not only an electronic product, but also an optical product. DLP and LCD projectors from different manufacturers may use the same or similar display devices and driving circuits, but their optical design has its own characteristics, thus forming different effects and styles. Generally speaking, the design of the projector has a high profit. It should pay attention to the optical, electronic, mechanical structure and other factors in order to achieve a good comprehensive effect. Neglecting the optical design of the projector and the quality of the optical components will be a fatal defect in the design of the projector. This is especially true for domestic 300000 pixel low-end video projectors. The resolution of 300000 pixels is not as good as that of 14 inch color TV sets. In the current technological development of color TV sets, few manufacturers have specially equipped 14 inch color TV sets with new technologies such as 3D and 5D image quality improvement circuits, progressive scanning, component input, etc. the main reason is that there is not much need for such a quality display. I believe that it is the key to design and measure the current domestic medium and low-grade projectors to do a good job in basic quality rather than pursuing fancy functions and concepts

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