Shanghai DIDON Industrial Co., Ltd.

What is The Brightness of “The Light Source”?

What is The Brightness of “The Light Source”?
As we all know, for the projector, brightness is an extremely important indicator. It is concern about the usage of the projectors.

However, recently in the projection market, especially in the home theater market, the concept of light source brightness has appeared.

But the questions is does the users really know the meaning of the light source brightness? Or the light source brightness will influence the choice of the users when they want to buy a projector for home theater?

The concept of the projector's brightness refers to the brightness when projected projector brightest white screen brightness in the normalized over the projection screen in the ideal environment.

Among them, the meaning of the ideal environment is "the light transmission from the lens to the projection screen does not cause any loss of brightness "; the meaning of "standardized ideal projection screen" is that the reflective of this curtain is 100%, and the angle of reflection is not.

There is an ineffective scattering, and the size of the reflective curtain just covers the projected image completely. Obviously, this so-called ideal state of brightness measurement can not be done, the actual method is to use environmental scattering coefficient, screen absorption coefficient, etc. to correct the error of the measurement system.

At the same time, the brightness of the projector under the standard definition is often consistent with the measured luminous flux at the lens .

What is the brightness of the light source ? Simply speaking, it is the total brightness of visible light produced by the light source as part of the illuminant .

If it is a blue laser fluorescent color wheel technology , then it is the total brightness of the blue laser ; if it is a mercury light source, then in addition to the red, green and blue primary color wavelengths, the light source brightness also includes other visible wavelengths (although the eyes are on the latter many sensitivity) is low; if RGB RGB LED light source, it is three primary color light emission brightness sum.

In a laser projector, the brightness of the light source is basically the sum of the brightness of the blue laser . The light emitted by the blue laser generally has to pass at least "light transmission", "expansion beam", "dissipation spot", "fluorescent pink wheel", "primary color wheel", "directional reflection lens", "DMD" light valve. , "Multi-group optical lens and projection lens composed of electro-mechanical structure ", and then presented on the projection screen. That is, the brightness ofthe light source becomes the projected brightness , and at least a dozen steps are processed in the middle. And each processing step will cause the brightness to drop.

There are few conditions of the brightness losses:

First, the transmittance of glass lenses, generally glass is slightly higher than 80% , the transmittance of optical glass is more than 94%. But even with such a high-projection glass, the total projection ability will drop to 73%.

Second, the operation of the DMD light valve. When DMD is working, the aperture ratio, transmittance, scattering rate, reflective, and system cleanliness all affect the brightness transfer, and none of these data can achieve 100% transmission brightness . This shows the effect of a DMD light valve on the brightness of the product .

Third, one of the biggest loss points of the light source brightness is actually a problem that many people ignore - the difference between the shape of the beam section produced by the light source and the mirror shape of the DMD light valve. DMD mirrors are generally rectangular at 4:3, 16:9, and 16:10. The cross section of the light source is generally circular and elliptical. In the simplest case, the effective illumination of a circular beam on a 4:3 shaped DMD is less than two-thirds, that is, nearly 40% of the brightness is lost at once .

Fourth, for laser fluorescent color wheel technology, the efficiency of fluorescence conversion must also be considered: the conversion efficiency of blue laser to green fluorescence, blue laser to red fluorescence. Among them, green light accounts for half of the white light energy, so the efficiency of green phosphor is a very big problem. For example, the quantum efficiency of most nit-ride green phosphors is between 70-90%.

In summary, if it is assumed that all of the optical efficiency of the system has reached 100% completely ideal state 5000 lumens of light source luminance of the laser projector, we can give an ideal luminance but also 2300+ lumens (considering aperture ratio, and DMD90% The light source beam section is poorly covered by a 16:9 DMD light valve and 80% phosphor efficiency).

In fact, there are no perfect optical system in the world: it has been listed above including "light transmission", "expansion beam", "dissipation spot", "primary color wheel", "directional reflection lens", "multi-group optics" .

A large number of optical structures, such as a projection lens composed of a lens and an electro-mechanical structure , each having an index of transmittance, scattering rate, absorptive, cleanliness, etc., affecting the transmission of brightness . These optical structures are sufficient to make the projector's light source brightness level a second fold. Thus, the projection luminance in fact only the light source brightness is about one-third, if the projection system is determined using single-chip DLP, blue laser fluorescent color wheel system, projection brightness will be in fear of the light source luminance of a quarter of it.

Finally, for projectors, the most efficient data should be ANSI brightness . The so-called ANSI brightness is actually optimize brightness , the brightness that is when the screen color performance A normal brightness when the specific evaluation we will take nine points to screen test. From a professional point of view, ANSI brightness and maximum brightness will be more meaningful to us.

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