Why ISF Calibration?

Welcome to Bjorn's ISF Calibration Service

Index

Introduction Viewing Environment Protective Screen, Light Box, Focus, Geometry & Convergence Black level, Picture level, Sharpness and SVM Color Temperature, White Balance, D6500 kelvin & the Black Body Curve Setting the temperature of white Color and Tint	Definition of kelvin Definition of Color Temperature Display Calibration Reports Customer comments

Hello everyone!

Bjorn's Audio, Video & Home Theater is offering Imaging Science Foundation certified Display Calibration services using a Sencore CP-5000 color analyzer, a Sencore 802BT Test Pattern Generator, an ISF optical comparator and various other equipment.

   If you want to improve the way your TV or monitor reproduces your favorite movie or graphics, you have come to the right place.

    The ISF Display Calibration process will bring your display device i.e. TV or monitor as close to the NTSC standard as possible within the limitations of your particular device. Proper calibration is essential so that the image you see is as close to the original put on film by the director as possible.

   With a well designed & calibrated display, a good transfer of a movie to laser disk or DVD and the correct viewing environment, you will be a long step closer to true "Home Theater".

   The information on this web site will help you determine if the ISF Display Calibration is the right thing to do and what will be done to your TV or monitor to get it to comply with the NTSC Standard.

Introduction

The display device in your home is the last link in a chain that started on a movie set or in a TV studio and is the least cared for in terms of insuring that the image that you see is anywhere close to the image on the movie set or in the studio.

One of the reasons for this is that on a show room floor there generally is a lot of ambient light which requires that the white level (contrast, picture etc.) of any given display device must be increased to compensate for this. Each manufacturer wants their set to "look good" next to the competing brand. So, where do they set the picture level? As high as possible without burning up the set!

One other reason is the setting of the color temperature. The temperature of white will be set to anywhere from 5000 kelvin on up. The actual temperature of white, as designated by NTSC is 6500 kelvin (D6500). Any higher and white will be more blue than white, any lower than and white will be more red than white. Watch for the Black Body Curve!

The settings your display device came with from the factory will utterly destroy the original image and do not let you see the movie, show etc. like it was produced.

Hence the need for a calibration!

The goal here is the reproduce the original image as close as possible within the confines of your viewing environment and within the limitations of your display device.

Before any calibration is performed, the signal path to the display device needs to be checked to insure that the interconnecting cables are of good quality and that the best possible signal source is used such as progressive scan DVD and HDTV.

My belief is that even with a modest budget for a display device it is still possible to get a satisfactory image that is close to the original by adhering to a few simple parameters:

   a.) tightly control the ambient light in the viewing area
   b.) get the display device calibrated
   c.) observe the correct viewing distance and position
   d.) get a comfortable chair
   e.) get a popcorn machine

One concept to keep in mind: there is no such thing as a perfect display device no matter how much money you spend on it. But we can come close.

Next we will look at what you can expect during a complete calibration.

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Viewing Environment

     Any ISF calibration really needs to start with the environment the display device is located in.

    The viewing environment is important for several reasons. For one, you want to re-create the Movie Theater experience in your home. The first thing that happens in a Movie Theater is the lights go down low with just a few sconces on close to the ceiling along the side walls. You want to be in the picture with no distractions from anything else in the room.

     The next reason to tightly control the ambient light in your viewing environment is the limited light output capability of your display device. Most display devices are not capable of delivering more than 20 - 25 foot lamberts (fl) without starting to bloom in the bright white areas of the picture and without over saturating the colors. The lower the light output, the higher the resolution of the display and the sharper the image, but you want at least 10 - 15 fl on the screen.

    With light output levels this low, you do not want any ambient light to fall onto the screen of the display device.

Ambient in this case is also all light that comes from the display device or screen itself and is being reflected off surfaces such as cabinets, table tops and walls in front of the screen back onto the screen, thus reducing the contrast ratio.

So, it is very important to treat the walls, floor and ceiling of the viewing environment with light absorbing materials such as theatrical black curtains for the walls and perhaps the ceiling and have dark carpeting.

When ambient light is tightly controlled, the eyes of the viewer do not have any other object to focus on but the plane of the screen. This will lead to eye fatigue during a prolonged viewing session.

To minimize this effect, the wall behind the display device should be illuminated with a 6500K light so that your eyes have a chance to focus on a different plane. Back lighting the display device will not be possible if you have a Front Projection TV (FPTV). In this case you will have to install sconces along the upper edge of the side walls outside of the field of view much like in a movie theater.

With the need to have no ambient light sources in your field of view that you can directly see, it becomes necessary to deal with the control panel lights on the equipment that drives your Home Theater. One easy way to do this is to have a curtain in front of all the equipment that can be pulled close during the viewing.

Viewing position
      Another consideration is the viewing position. In general the prime viewing position is in the middle of the display device about 1.5 - 3 times the width of the screen back. Again, you want to be in the picture, you want to be part of the movie.

Viewing angle
    The viewing angle comes into play with Rear Projection TVs (RPTV) where the vertical viewing angle is limited to about +/- 15 degrees.

    To get the most light delivered to you eyes, the RPTV may have to tilted forward or the seating area may have to be raised.

Magnetic fields
Even tough most speakers today are "magnetically shielded" there is no way to completely eliminate the magnetic field from a speaker. To avoid any convergence shift etc., speakers should be placed at least two feet away from the CRT(s).

Also, I have seen cases with RPTVs that when the set was moved the convergence changed. This was caused by the steel in the concrete slab of the house being magnetized. Once the set has been adjusted, it should not be moved at all.

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Protective Screen, Light Box, Focus, Geometry & Convergence#

After the viewing environment has been properly adjusted, it is now time to address the display device itself.

Most steps described below apply for all types of display devices, rear projection, front projection, direct view, etc. I will use a rear projection TV as an example. RPTV and FPTV have three individual CRTs, red, green and blue.

     Before any adjustments can be performed, the display device needs to warm up to normal operating temperature for about 20 minutes with the white level set to about 50% of what the initial factory setting is.

Protective Screen
     While the set is warming up, the protective screen on RPTVs should be removed because it reflects any ambient light in the room if there still is any. The protective screen on some RPTVs is actually more opaque then you would expect from clear plastic or glass, thereby acting as a filter to prevent some ambient light from reaching the screen. But this also causes a reduction in light output from the set as well.

     Some manufactures claim that the protective or contrast screen improves black level and contrast. This true in an viewing environment with lots of ambient light. Without the protective/contrast screen the light falling onto the lenticular sheet of a RPTV will wash out the picture under these viewing conditions.

Since we will be dealing with a controlled viewing environment with very little ambient light, the protective/contrast screen will need to come off.

     The only reason that the protective screen should remain on the set is if there is concern that small children may damage the lenticular sheet and/or the fresnel lens behind the protective screen.

Light Box
    While the screen assembly is off for the removal of the protective screen, we need to coat the inside of the light box with a black, light absorbing material that will kill any un-wanted reflections from the painted wood surfaces and the hardware that holds the mirror and tube assembly.

Focus
     The next step will be to focus the image. This includes optical and electrical focus to improve the center focus as well as scheimflug adjustment, if available, to improve the edge focus if necessary.

Geometry, Over Scan & Convergence
     After focusing, the geometry and convergence of the image need to be adjusted. Geometry is the overall shape and size of the image. With geometry, green is used as the reference to which red and blue are converged to make one image.

Geometry includes the need to minimize over scan. A certain amount of over scan is necessary so that the information inserted at the top of the picture is not seen. Over scan is not necessary with DVD but all but the most expansive data and graphic displays are capable to display the entire picture without noticeable geometric and/or convergence distortions

Good geometry insures that the entire image is level, centered and sized properly. If all three images are well focused and converged, bleeding of colors around edges or lettering will be minimal if any at all depending on the make and model of your set.

Depending on the make and model of your RPTV, focusing, geometry and convergence may take up to 80% of the entire calibration process.

A note on two piece front or rear projection displays: the mechanical relationship between the projector and the screen is of up most importance since they are not housed in a pre fabricated enclosure and mechanical alignment errors can easily occur which may make focusing, alignment and convergence difficult and in some cases even impossible.

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Black level, Picture level, Sharpness and SVM

Now that we have a properly shaped picture we can start to adjust the picture quality.

Black Level
First item on the list is the black level (brightness). The purpose of this adjustment is to be able to reproduce the color black or the absence of light correctly.

If black level is set too high, the picture will appear to be washed out and if adjusted too low you will not be able to see details in the dark areas of the picture.

Some display devices are not able to hold black very well. This normally means that when black is set correctly with a low APL (Average Picture Level) Pluge and then a high APL Pluge is displayed, the black level will slightly rise. In some sets this is adjustable and a good set will have as little shift in black level as possible.

Picture Level
The next adjustment to be made is the picture level (contrast). This will set the maximum peak light output of your display device. Ideally this will be between 15 & 25 fl (foot lambert) but at least 10 fl.

     However, some lower end HD ready and non HD ready display devices may not be able to produce 15 - 30fl without overly saturating colors & without distorting the picture or causing bright white images to go out of focus.

     If that is the case, the maximum picture level will have to be adjusted so that there no distortion of the picture when a 100% white window is displayed.

     With Front Projection TV's (FPTV) the maximum useable light output is limited by the size of the screen. The larger the screen, the lower the amount of reflected light from the screen.

     For most CRT based FPTV's the maximum screen width for a none perforated screen will be around 7 feet. If you want a screen wider than that, you will have to stack two or even three projectors in order to get a 15 fl light return from a none perforated screen.

     The goal is to get a picture in terms of light output that is at least equal to that in a movie theater which is 10 - 15 fl. So you may have to limit the screen width to get the desired light output. Bigger is not always better.

Sharpness and SVM
    Before the sharpness is adjusted, scan velocity modulation needs to be turned off. I will actually defeat/disable the SVM as the first step in any calibration! The SVM is a circuit that gives all dark objects on a light background an over accentuated look and widens these objects at the same time which reduces picture resolution.

This is especially important on direct view sets. On some Sony sets, the SVM widens the objects to almost twice the original width!

This is a wholly unwanted effect. No professional display device has this and neither should yours.

To obtain a sharp picture the sharpness control needs to be set to a level where the aforementioned dark objects have a nice crisp edge without any white shadows next to them.

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Color Temperature, White Balance, D6500K & the Black Body Curve

Color Temperature and White Balance
Color Temperature and White Balance are terms that are inter-related in meaning. Color Temperature is the actual kelvin measurement of what is represented as white within a picture. White Balance refers to the balance or proportion of Red and Blue content present in a white signal.

Actual kelvin measurements are obtained by super heating a carbon block until it begins to glow and emit light. The white light that is radiated by the carbon when it is heated to 5,500K becomes its reference on the corresponding color spectrum diagram. Depending on your reference, a color temperature of 5,500K will produce a slightly reddish cast in white images, yielding what is commonly referred to as a "warmer picture". This setting is utilized as the white balance reference in black and white film as it provides warm, natural flesh tones.

Similarly, white light emitted by carbon when it is super heated to 10,500K will reflect an increased proportion of blue within the white image. White images will appear bright, powerful and bold.

Interestingly, the human eye is tremendously adaptive. Viewed alone, a white balance setting of D6500K will appear to provide pure white images in the picture. Conversely, a white balance setting of 10,500K will appear to be similarly pure. It is only when there is some visible reference point that the human eye detects the presence of blue or red within the white signal.

NTSC Standard

The North American Television Standards Committee assigned D6500K as the reference standard for the white signal, and some in the creative community conform their movies to this measure, D6500K is the color of white on an overcast day. Viewed in a low ambient light condition of a Home Theater, whites appear natural and an overall warmth is present in the picture. However, in a medium to brightly lit room, lower color temperature settings are less versatile and the brightness performance is generally perceived as undesirable. The picture appears darker and reflects low contrast.

reprinted in part from Toshiba America

Black Body Curve

The reason we have to re-calibrate display devices is because the industry decided to deviate from the NTSC standard.

Why have a standard when the manufacturers of display devices are not adhering to it?

In the past few years some manufacturers, namely Mitsubishi and Sony that I know of, have started to include NTSC 6500 kelvin color temperature settings. From what I have seen and measured it turns out to be anywhere but D6500K. So they still don't have it right.

If the manufacturers would have stuck to the NTSC standard we all would be watching movies in a dark, dimly lit room with a properly calibrated set all along.

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Setting the temperature of white
To get your display device to comply with the 6500K (D6500) requirement Of NTSC, we need to use a calibrated reference like an optical comparator or a color analyzer to set the color temperature. There is only one point where light is white and that is 6500K (D6500) or the coordinates x=0.3127 and y=0.3290 on the blackbody curve. If the color temperature is set any higher, white will become more blue and any lower it will become more red.

With the proper settings of white level and color temperature, display devices would also last a lot longer since they are not being operated on the ragged edge of self destruction all the time. Maybe there is a conspiracy here? I have seen a number of sets lately that have been used an average amount of time a day without having been calibrated that needed to be re-tubed after only 4 to 5 years in service. There is much to be gained here from calibrating the display device: The image will look like it was intended to look and your display device will last quite a bit longer.

In the early 80's, I used my eyes to set the color temperature and found out that this worked fairly well considering that I did not have a reference to work with.

In 1992, I started using an ISF optical comparator with which I had very good results. The only short fall with the optical comparator is that I was only able to set the color temperature to D6500K and I was not able to print out results.

Times have changed and we now have a calibrated Sencore CP-5000 with which to set the color temperature. With this instrument I will be able to be a lot more precise and document what the condition of your display device was before and after any adjustments were made. I will also be able to store these settings so that we are able to track the wear on the CRT's in your display device.

Recently we have added a Sencore 802BT Test Pattern Generator that provides us with signals in the following formats: HDTV, SDTV, NTSC, PAL, VESA (computer monitor formats).

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Color and Tint

Now that we have an image that has the correct color of white at the correct level, we can add color to the image. Up to this point we have been dealing with black and white images. Color and tint are set with the blue only color bars.. If the display device allows, the color decoder may also be adjusted while displaying the red only & green only color bars.

When the color intensity is adjusted correctly using the blue only color bars, I find that the red objects in the images on a lot of display devices are way over saturated. If the display device does not have adjustments to calibrate the color decoder, then the only remedy is to decrease the overall color intensity to where reds stop blooming and people have a natural looking skin tone.

In the case of component video from DVD and HDTV some correction is possible with an attenuator inserted into the red signal coming from the source.

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Page updated: Sunday, March 13, 2005 05:59:58 PM -0600