About LED light engine
When considering LED light, just remember two things; small light emitting area and high power rate
Smaller emitting area will give you sharper images as it becomes more similar to a point source which is ideal. Otherwise, you will end up with blurry edges. The conventional DIY LED light engines has quite large emitting area. If you are stick to it, you cannot make perfect focus forever. It works ok only when you make FHD or less-resolutioned beamers.
High power rate will obviously will give you more bright image. As the conventional LCD is quite dark, you should have enough light power to penetrate much of light as much as possible. Meanwhile, you will burn up your LCD unless you do cool down LCD properly.
Considering above factors, I've tested several LEDs.
First, I tested CBT-140 (from Luminus) which has the smallest emitting area in the market (used in my 2k beamer). However, the power is rated at only around 100 watt, so the brightness is not enough though the image was very sharp.
Second, I switched to 200 watt LED which has no brand, made by the taobao seller I mentioned. It gives enough brightness under dark environment, but I wanted it to have smaller emitting area for sharper images.
Luckly, the taobao seller told me that he could make 200 watt LED, but smaller emitting area than the second one. As shown in the pic, the emitting area is quite comparable to CBT-140. The brightness is satisfactory and the image quality is also satisfactory. SSD-90 is engraved on the LED base, but it is not SSD-90. It is the modified version of SSD-90, I guess.
About condenser lens
You may be curious why I used the rectangular-shaped condenser lens.
In the conventional-DIY-beamer way, an aspherical condenser lens is used. Yeah, I tried F78 condenser lens which is commonly used in 7" LCD with F140 back fresnel lens. The lens can be also used with the comination of 10.1" (but 16:10 ratio) with F140 back fresnel lens.
In short, I could not use the combination; F78 condenser lens / F140 back fresnel lens, because the LED light does not fully cover the active area of the 4k LCD. In addition, I could not find 0.3mm (or less) pitched F140 fresnel lens, so the image quality was not good.
That's why I moved to the rectangular-shaped condenser lens. This condenser lens concentrates more light onto the LCD, but the light passage and the focal length are not given analytically.
So? I should find out the best optics positions by many many experiments. If too close to the back fresnel lens, you will see yellowich edges. If too far, you will see bluewish edges. At certain points, the brightness is quite uneven.
This little devil made me stay many nights through several weeks. The final lens positions are quite far from our DIY-beamer knowledges. Anyway, do not worry, just use my drawings.
Some notes on distances between LCD and fresnel lenses
I just want to give you my experience on the lenses and LCD spacings.
If the LCD is too close to the back fresnel lens, then the grooves will be projected on the screen.
If the LCD is far from the front fresnel lens, the image becomes blurry especially at edges. However, if too close, then discontinous-like images are projected on the screen due to the grooves on the front fresnel lens.
In the mean time, the projection lens quality is important. By rule of thumb, the focal length of projection lens is chosen in the range about +20mm of the front fresnel lens. So, in this case, F240 would be good. However, that is not the only factor to be considered, Field of view (FOV) is another dominant factor in the image quality. Testing many projection lenses, I found out that F260 gives me the best result among them.
Yes, many variables exist. That's why I did many tests.