Something New, Something Old
The Art of Presenting Grayscale Calibration Work
To The Public
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Michael TLV writes:
Up until now, magazines and other information sources on Grayscale
calibration presented results to a small, but curious public in the form of Grayscale
graphs charting the colour temperature versus various intensities of gray. A sample of such a chart is shown below. We see these in all the home theatre magazines and
some ISF calibrators provide their customers with something similar called the ISF
calibration report where the primary focus is on the big before versus after graph.
Now by itself, the graph is a nice visual
presentation of the TVs grayscale tracking before calibration and after calibration. Now here comes the problem. The grayscale graph is a gross simplification of
the grayscale calibration process and the science behind what is being done. (Although
actually doing a grayscale is not that difficult with the proper tools.) It is presented in this graphical format because
it is feared that the public cannot grasp anything slightly more complex about grayscale
calibration and the basic theory behind it.
I bring this up because on not so rare occasions,
the information presented on the graph is often extremely misleading. I have described the grayscale calibration graph
to my clients as being something akin to a 2-D representation of something that is 3-D. I draw a sphere on a piece of paper and it looks
like a circle. As a result, just because
something looks close on a colourful graph may not actually mean anything. A blunt and extreme example that I like to use is
to place your thumb into the night sky right next to the full moon.
From a two dimensional perspective, your thumb is
now as large as the moon and possibly bigger. As
such, can we walk away and conclude that the moon is not very large at all? In a two dimensional universe, the answer would be
yes. Of course we inhabit a
three dimensional universe so the answer is no, because we know that there is
also a Z axis in three dimensional space. X,Y,Z coordinates in real space. Width, Height and Depth
The grayscale charts that we see in magazines
simply do not do justice to the grayscale calibration theory. And often times, a client will misinterpret these
same grayscale graphs and conclude that his TV was so closely tracking grayscale out of
the box than he did not really need to hire you in the first place. Yes, there are cases where some TVs really are
close from the factory, but this has been fairly rare and so far, continues to be rare. Its just that sinking feeling that one
gets when his client misunderstands the grayscale chart information. When this happens, you can end up with an unhappy
client and everything goes sour on you (doubtful, but possible).
want to bring in a few exhibits here that are gross exaggerations, but do, hopefully, get
the point across. The grayscale calibration chart sample below represents the major
deficiency of the current graphical presentation method. We have a case where the
pre-calibration grayscale tracking appears to closely resemble the post calibration
The reaction from the uneducated public would be
that the TV in question was pretty accurate out of the box and that the calibrator
probably did not have to do very much if anything at all to fix this. This is what a graph shows you
and this is
what the image actually looked like before calibration versus after calibration.
It is nearly impossible for the
home theatre publications to present to us what the actual image looks like on the TV. Hence we have the grayscale calibration charts
that are easy to translate to print. Now the
pre-calibration image of the resident Furry Pig is quite simply
all wrong. It is too green.
The post calibrated image is pretty much where the image should be and yet
the graph doesnt show this at all. This
is why the calibrated grayscale graph that we see in magazines have the potential to also
be terribly misleading. Good data presented
the wrong way can lead to unfortunate and erroneous conclusions.
Of course the fact that I am presenting images on
a web site also introduces a plethora of potential errors.
The irony of this fact did dawn on me.
All images are therefore presented for illustration purposes only. I have to figure that even the poorest tuned
computer monitors out there will at least be able to show the reader that the two images
of the Furry Pig look distinctly different. How
that difference is manifested on the screen, I have no idea.
From the two-dimensional perspective, it is the
same as your thumb being as big as the moon. Now
imagine that you can see 12 inches behind the graph and 12 inches in front of it. The further behind the graph you go, the greener
your image becomes. The further in front you
get, the more red/purple the image becomes. The
green pre-calibration Furry Pig is actually located six inches behind the graph. The post calibration Furry Pig is located on the
graph itself. The idea in calibration is to
get the curve onto the surface of the graph itself too, not in front or in back. This is what D6500K is all about. There are lots of 6500K readings both in front of
the chart as well as behind it. Sometimes,
the curious public loses sight of this and simply gets focused on the magic 6500K number
thinking that it is only the number 6500 that is all important.
So the big question is, how do we present
the public with something that is more informative and gives a better representation of
the actual grayscale calibration process and yet does not make them roll their eyes in
confusion. There must be some way to properly quantify how much better the grayscale
tracking is after the calibration process. Presenting the client with a graph like
in Sample #2 is undesirable and has the potential to send the wrong message. It
ultimately tells the client little to nothing in terms of pre-calibration and post
calibration. (Although in the end, we all let our eyes be the judge of how good or
bad things really are and someone passing off that green Furry Pig as being a properly
calibrated image would never get away with it regardless of how the numbers read. This is
a good thing. =)
The quest is to find a better way of expressing the way the TV was before and after a
calibration session when it comes to grayscale tracking while keeping it simple enough for
people (re: enthusiast) to understand. Enter the following spreadsheet chart.
(The M-TLV grayscale calibration report)
The M-TLV Grayscale Calibration Report
Click on table to enlarge
Although this is nowhere near as
colourful as the typical grayscale tracking chart found in magazines, it is easy to use
and far more accurate in terms of telling the client just how different his
pre-calibration grayscale was compared to the post calibration grayscale. (Actually, most magazines present the graphs in
black and white so there are no colours to speak of anyway.) It provides a much better view of just how good or
bad a clients grayscale tracking was. The
sheet still provides information at each light intensity tested, (IRE window box patterns)
just like the graphs, but instead of the colour temperature reading, we get X and Y
coordinates in what is known as colour space. This is how we actually do it as opposed to
looking for that 6500K colour temperature.
D6500K on the CIE Chart
A quick and simple explanation of the CIE chromaticity
chart is that it represents all the colours that the human eye can pretty much see. Our eyes can see many more colours than what a TV
is capable of producing though or even what a magazine is capable of printing. There is a specific point on this chart that
represents what neutral colour information is, a point where black and white information
is considered to be absolutely neutral. (not
tinted blue or green or red.) This point on
the graph is denoted by the coordinates X=0.313, Y=0.329.
You do not have to burn these numbers into your head. Just know that they represent the D6500K point
that we strive to achieve.
The only thing the end user needs to be aware of
on the chart is the last column of numbers. The
percentages are nice and easy to understand. The
big 93.9% figure tells the client how much more accurate his grayscale
tracking is compared to when you started. It
much better quantifies the work that you do for the client.
The more whacked out the TV is compared to the post calibration state, the
better the overall percentage number. The
lower the percentage number, the closer the TV was to being accurate. So if the overall improvement is 10%
know the set was pretty accurate to begin with (or the calibrator is terribly incompetent
or there is a major problem with the TV. ) The
calibrator is still expected to get the grayscale as good as possible.
I encountered a 1997 Sony 61 XBR RPTV
recently and the set tracked grayscale fairly poorly and it was near impossible to even
get something close to correct. The end
result showed an improvement of 34% compared to the pre-calibration result so low numbers
do not have to imply that the TV was more accurate.
final percentage numbers cannot be used to compare grayscale tracking between two
different TVs though. The numbers are
only relevant for the display that the values were taken from. A 95% improvement on one display is not
necessarily better than a 50% improvement on a second unit.
The sheet does, however, present a set of numbers than can be compared to other
TVs. The directly comparable numbers
are the Distance to D6500 values. If
my pre-calibration totals are less then yours, then my set was closer to optimal at the
start. If my post calibration totals are more
than yours, then my set tracks worse than yours
regardless of the percent
improvement figure. Although not normally
used by the client, the information is still presented.
This actually permits two separate people to compare notes
if the same
calibrator worked on both their TVs.
The Bias-Tint column can be used by the client to
judge the imperfections of the grayscale in the display.
At any of the tested IRE windows, this box tells them exactly what type of
slight tint they may be seeing. At the low
IRE windows, it is better to have the tint be blue than green, but if the improvement is
really close, then do not worry about it. Let
your eyes be the final judge.
This article is not about teaching people to
properly calibrate the grayscale on the TV. It
is about presenting the information to the client in a more meaningful manner and yet not
totally confusing the client. While
presenting much more information than the client needs, the tale of the tape is in the
The breakdown of the sheet is also provided with
all the graphics design work credited to Errol. The
spreadsheet also includes a number of comments about the various columns in terms of what
I am trying to achieve.
I have made a copy of the spreadsheet available
for download at this site for
those people / calibrators curious to try this manner of presentation out. You can also e-mail
me directly and I will send you a free copy.
I will be testing out this output format for my
clients over the next few months. This form
also has potential to be used in conjunction with the standard ISF report format. It merely replaces the graph on the report. Unfortunately, the formulas that are part of the
spread sheet require that values be entered into a spreadsheet first.