LCOS vs DLP, .65″ DC3 DMD vs .95″ DC3 DMD – The Sharpness Debate

June 21, 2009

Frequent debate and speculation has centered around the sharpness differences between DLP and LCOS display technologies.  Unfortunately these comparisons are complicated by factors such as 3-chip panel convergence errors, lens quality and chip size.  Often, these debates have compared images from single chip DLPs with no convergence errors, larger chip sizes (.95″ for most commercial DMDs) and expensive optics.  By comparison, LCOS chips tend to be smaller (.7″ JVC or .6″ Sony), with less than perfect panel registration (owing to their 3-chip design) and optics befitting their lower price points. 

With the recent rollout of the smaller .65″ TI DC3 DMD which has been incorporated into Optoma’s new HD8200 , we can equalize one of the variables in the sharpness equation and compare images from similar sized chips directly.  Unfortunately MTF(sys) = MTF (lens) + MTF (chip) so without knowing the MTF (sharpness) of the lens it’s impossible to say conclusively that sharpness differences in an image comparison are solely due to differences in the technology.  But this is an interesting comparison nonetheless and it does provide one data point even if the results are not conclusive.  

For this comparison we’re going to examine the sharpness of 3 projectors.  For DLP projectors we are going to examine images from the new Optoma HD8200 with the new .65″ DC3 and also the Planar 8150 which has the older and larger .95″ DC3 DMD.  For LCOS we will examine a JVC RS20 with very good convergence between red and green but with blue off by approximately half a pixel.  Since humans are much less sensitive to blue and it is weighed much less than the other two colors (~11%), the blue alignment issue is less significant than if one of the other two colors were misaligned. 

First Up – Optoma 8200 vs JVC RS20

First let’s take a look at images of both projectors from normal viewing distance using the Sony Blu-Ray Easter Egg resolution test pattern:  

Optoma HD8200 (click to enlarge)

JVC RS20 (click to enlarge)

 On initial examination, both images look very similar and seem to resolve this pattern well.  The biggest difference in the images is the cyan and magenta tint on the RS20 that affects some of the areas of the test pattern, particularly the zone plate portion of the test pattern which is the area at the corners with radiating lines looking like spokes in a wheel.  This is most likely due to color bleeding from the blue misconvergence.  Unfortunately there is also a slight tint to the white in these photos that wasn’t in the actual images and was introduced by the camera.  These particular photos were taken with a Nikon Coolpix camera, but later photos in this post are taken with increasingly higher quality cameras (a Panasonic Lumix DSLR and later a Canon 50D DSLR). 

Optoma (click to enlarge)

RS20 (click to enlarge)

Examining these two images closely, both projectors look remarkably similar as far as resolution, although the RS20 seems to have a slight advantage in detail and sharpness, with the drawback of blue color bleeding from the misconvergence of blue.   

RS20 vs Optoma – a more rigorous look

Next, let’s take a closer look at the pixel definition of both projectors.  In this example we’ll use a test pattern consisting of single pixel alternating on/off columns.  What makes this pattern interesting  is that this represents the maximum spatial frequency of a digital display and this is how MTF is traditionally measured on a digital microdisplay.  A detailed discussion of MTF is beyond the scope of this post and rather than get into the details here I’ll just mention a few points that are important to keep in mind.

The formula for MTF is:  MTF = Lmax – Lmin / (Lmax + Lmin).  

(edit: 6/30/09 – The general form for MTF is complex, but this formula works for a specific spatial frequency.  A discussion of this can be found here: http://www.revisemri.com/questions/equip_qa/measuring_mtf )

MTF is a contrast measurement relating the luminance from the peak white (Lmax) from the “on” pixels to the darkest blacks in the valley between the on pixels (Lmin).  Classic MTF measurements use a sinusoidal waveform of varying frequencies from low frequency to high frequency and MTF is calculated for each of these frequencies and plotted on a graph with the Y-axis being MTF as a percentage from 0 to 100% and the X-axis being the frequency.  Because the equation is normalized, MTF graphs typically start off at a high percentage (100%) and then roll off at the highest frequencies.  With a digital panel, the alternating on/off pixel pattern provides the highest spatial frequency and therefore the MTF will be at it’s lowest point on the curve with this pattern.  For this reason, this alternating pattern is the most useful pattern used for comparison purposes and is often referred to as the Nyquist cut-off frequency. 

For these next comparisons, we’ll convert the images to a greyscale, which makes it easier to compare relative luminance differences.  Any color details including the blue misconvergence of the RS20 is not thrown away from these photos but rather is added to the luminance values in the greyscale.  I should also mention that all of the photos in this post were taken directly off of a Stewart Studiotek screen, so any small “sparklies” and texture that you see are due to the screen.  As before, the camera used, camera distance and camera settings are all the same between this next comparison with the only difference being a slight change of exposure to equalize the brightness differences between projectors. 

Optoma (Click to enlarge)

RS20 click to enlarge)

With the single alternating pixel pattern above, it appears that the RS20 seems to have a sharpness advantage.   Individual pixels however, seem to have more definition on the Optoma.  In particular the vertical lines on the RS20 seem to merge together.  But, overall however, the pixel brighness of the peaks is brighter on the RS20 and the blacks in the valley (off pixels) are darker.  Using the MTF equation above would indicate that the RS20 has higher MTF at the 1080p Nyquist Limit.    This can be more easily seen when we superimpose portions of both images in the photo below.

Optoma (top) JVC RS20 (bottom) superimposed

Unfortunately a digital camera captures only relative luminance differences in an image and not absolute luminance, so without more information (like the camera gamma), it’s probably not possible to calculate MTF directly.  We can, however, determine relative luminance differences and this in turn tells us which projector has the higher MTF even if we do not know what this number is exactly. 

Since we are using a digital camera to measure sharpness differences we can also go a step further and rather than visually inspect just a few pixelsm we can look at the histogram distribution of all of the pixels of each image.  These are shown below and were obtained from Photoshop: 

Optoma histogram

RS20 histogram

In these histograms, the X-axis represents pixel brightness with dark at the left and white at the right.  The Y-axis represents the total number of pixels in the image that share the same brightness.  Ideally, with a perfect display, one with high MTF, we would see two vertical spikes, one representing bright (on) pixels and another spike of black (off) pixels and with little spreading between them.  The important thing (from a resolution standpoint) when viewing these histograms is to have as much distance separation as possible between the spikes.  Looking at the images above, we see that the RS20 clearly has more separation between the spikes even though the bright spike representing the spread of “on” pixels is broader and less defined than the dark (off pixels) spikes. 

Next Up – Planar 8150 vs JVC RS20

 The next comparison is between the same RS20 and the Planar 8150 which uses the larger and more common .95″ DC3 DMD.   For this comparison we will use a Canon 50D camera which yields a little bit more resolution in the photographs and  higher pixel density (15.1M Pixels). 

Let’s start off by examining real world images as we did with the Optoma and RS20 comparison.  The images below are from a dialog box in the Windows desktop: 

Planar (click to enlarge)

RS20 desktop (click to enlarge)

From these photos the Planar is clearly sharper than the RS20 and by inference also sharper than the Optoma HD8200, although the pixel grid and SDE (screen door effect) is also much more noticeable.   We can see the SDE differences even by looking at a solid white field which is shown below:

Planar White (click to enlarge)

RS20 white (click to enlarge)

RS20 vs Planar a more rigorous look

The first comparison will be of the now familiar, single pixel, on/off, “multiburst” test pattern.  As we did earlier, we will convert these images to a greyscale without throwing away the contribution to Luma by whatever colors are found in the images (including the blue misconvergence of the RS20).  As was done before, the camera position and zoom settings haven’t changed between these photos and the only difference is a slight change in exposure, to correct for the brightness difference between projectors.  It should also be mentioned that slight exposure differences will not affect the relative differences between white and dark and only serve to shift the whole curve towards bright or dark without affecting the shape of the curve and therefore the peaks needed for relative MTF comparison.   Therefore, so long as the exposure isn’t overexposed or underexposed (which would clip the whites or the blacks), this technique will work well and we can use small exposure changes to correct for projector brightnesses.

Planar (Click to enlarge)

RS20 (click to enlarge)

 Next, let’s take an even closer look at each of the test patterns from above.  Note:  The images below were captured by optically zooming the lens and weren’t resized in photoshop: 

Planar closeup (click to enlarge)

RS20 Closeup (click to enlarge)

Planar and RS20 closeups superimposed (Planar on the bottom)

 

Planar RGB Histogram

 

Next is the JVC RGB histogram.   We can see how well the green and red histograms overlap.  The blue histogram, however at least on this particular projector, shows less sharpness in blue compared to the other colors. 

 

 

RS20 Histogram (RGB)

 

.65″ DC3 vs .95″ DC3 DMD

 The data for one projector using a .65″ DC3 and another using a .95″ DC3 have been posted above in conjunction with the RS20 being used as a comparison for each.  It’s left as an exercise to the reader to directly compare the two.   As was mentioned earlier, it is impossible to know if the large differences in sharpness that we are seeing is due to differences in the lens or due to differences in the chip sizes because we are seeing the combination of both.  I will say though that the lens in the Optoma did not seem to be particularly bad.  It did not suffer from too much chromatic aberration for example, and it seemed to be fairly even across the field, although it was a little less so in this respect than the lens on the RS0 or the Planar. 

 Summary

In this post we examined the sharpness differences of an interesting mix of 3 projectors -  Two DLPs using two different DMD sizes (.65″ and .95″) and one LCOS with a .7″ DILA panel.  All three projectors resolved 1080p test patterns well and none seemed to suffer any deficiencies in sharpness with test patterns or video images.  The two projectors that were the most similar in terms of resolution and SDE also happened to have similar chip sizes.  Most surprisingly these same two projectors  (The Optoma HD8200 and the JVC RS20) were the most dissimilar in terms of display technology with one being single chip DLP based and the other being a 3-chip LCOS design.   By comparison, the Planar 8150 was significantly sharper than the other two when compared with test patterns.  With video content the sharpness advantage of the 8150 was also noticeable although perhaps not as striking.  SDE on the 8150 was also much more noticeable unfortunately.  Color bleeding on the RS20 due to it’s 3-chip design was also apparent.  One thing that is clear from these comparisons is that there is much more to sharpness than the panel technology alone.  We have shown for example how a 3-panel LCOS design can be just as sharp or sharper than a given 1-chip DLP design.

  

In this post we also showed how easily the CCD in a ubiquitous DSLR can be used to determine relative sharpness differences.   We used two DSLRs, a less expensive Panasonic Lumix and a more expensive Canon 50D and so long as the projector pixel’s were oversampled with the camera pixels, the results seemed to track very well even with different positions, lens and camera settings. 

 

There is an implication in this post that the panel size alone can have a large impact on image resolution.  More data needs to be collected to prove this implication, but if true it means that the quality of optics as chip sizes are reduced will be increasingly important.  Many manufacturers have shown a desire to reduce chip sizes as a way to lower costs, and we have seen TI do this with the .65″ DC3 DMD used in this comparison, we’ve also read published reports that JVC will go to a .45″ 4th generation 1920×1080 display chip.  As these new, smaller chips are released it will be interesting to see how much image sharpness will be affected.  It may become the case that any cost savings due to smaller chip sizes will be eclipsed by the increased costs required by new demands on the optical and lens system.