Lumagen Tech Tip 2 Gamma, Grayscale and Gamut Calibration
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1. can be used to correct the color of gray often called Grayscale or White Balance Gamma black level and white level The Radiance can independently correct the primary and secondary color points and colors within the RGB color cube on displays where the color points are outside the CIE color triangle and now starting with software rev 081413 on the 20XX series up to 729 points throughout the color space 2013 Lumagen Inc l Radiance Tech Tip 2 Rev 5 Many displays and video processors claim to have CMS color controls but instead really only have Color Hue Offset controls Color Hue Offset controls should not be used to try to correct the primary colors because they operate in the wrong color space Other manufactures attempt to calibrate the primaries using HLS color space controls These controls also operate in a non linear color space and so provide incorrect results Set these display controls to neutral before starting a new calibration Lumagen uses a linear Gamma RGB color palette for CMS correction Lumagen believes that linear Gamma RGB color space the native color space for displays and projectors is the correct color space to use for color correction This allows the primary and secondary colors to be corrected while maintaining correct color interpolation Figure 2 CIE chart after calibration Original Radiance 8 point CMS SW Rev 050312 and earlier The original Radiance 8 point color gamut2. the primary and secondary points are at or outside the CIE color triangle for the desired color space Some comments on color probes There is great debate about the accuracy of color probes in measuring primary and secondary colors For tri stimulus probes the accuracy is mainly determined by the color filter s response curve and having a specific set of offsets for each display type For spectral radiometers the number of sample points often determines accuracy and low light performance Lumagen recommends caution when using the results obtained from color probes For example a color probe may be inaccurate at 10 luminance and using it to calibrate at this level would therefore give inaccurate results A number of commercially available probes are not as accurate as many believe for measuring primary and secondary colors or taking low light readings Higher accuracy probes can cost 10K and up Lumagen believes that it is better to err on the side of slight over saturation rather than to allow potential under saturation of the primary or secondary points Given that there is a potentially unknown error in the probe s x y point reading it may be that having readings slightly outside the CIE triangle will provide better visual results than inaccurate readings that exactly match the CIE chart Measure the x y and Y values for all the primary and secondary colors and white Enter the measured white Y values into a lumina
3. with the x y and Y of the CIE diagram So you need to iterate between adjusting x y and adjusting Y for each point Save your new settings by pressing Menu Save OK OK OK Remember to save your Radiance settings after any changes or you will lose your work when the unit powers down 2013 Lumagen Inc 6 Radiance Tech Tip 2 Rev 5 23 Take post calibration readings to make sure you are within target error values dE s 24 Watch and judge real video material It is possible for a calibration to measure well but provide poor image quality Using the Radiance this is generally not an issue But this step is always a good idea and a way to sit back and enjoy the great image quality you ve achieved using your Radiance 2013 Lumagen Inc 7 Radiance Tech Tip 2 Rev 5
4. Lumagen Tech Tip 2 Gamma Grayscale and Gamut Calibration Introduction Many displays have over saturated colors For these displays the primary and secondary color coordinates lie outside the CIE color triangle The CIE diagram in Figure 1 shows an example of a display with over saturated colors Many displays and projectors with this anomaly don t have a way to correct the problem Other displays use an inappropriate method in an attempt to resolve this issue which also leads to color errors Such color inaccuracy is most noticeable in flesh tones and natural colored objects such as grass In the Radiance series Lumagen uses an advanced linear Gamma 3D color palette to independently correct the primary and secondary color points and has recently added the ability in the 20XX series to independently correct a total of 729 points 9x9x9 throughout the linear Gamma RGB color cube Because of this the Radiance can accurately correct colors providing a linear response including mitigating the non linear response exhibited by many displays and projectors Figure 1 CIE chart with over saturated colors Color Management This tutorial is intended for experienced calibrators If you don t have proper training and an accurate color probe Lumagen recommends having your projector display calibrated by a calibration expert The Lumagen Radiance Series of video processors has an advanced Color Management System CMS The CMS
5. alette processes the image Finally the Gamma pre emphasis is reapplied to the video Note You do not need to convert non linear colors into linear colorspace 2013 Lumagen Inc 2 Radiance Tech Tip 2 Rev 5 The values are converted to non linear space internally for use in the 3D gamut Values outside the 0 to 100 range are interpolated to provide good results for below black and above white Transitioning to 091612 and Later Software When updating software from pre 091612 software releases the original 8 point CMS values are automatically updated to preserve the original calibration Since the older software and new software control different points in the RGB color cube the numbers in the menu are different but will provide essentially the same results Video Pipeline For new software 091612 and later the 3D RGB palette and 1D RGB grayscale palette are in series The video pipeline first goes through the 3D RGB palette and then goes through the 1D grayscale palette Because of this order you need to calibrate the Gamma Factor then the 11 or 21 point grayscale Gamma with default values in the 3D RGB palette Then when you calibrate the gamut 3D RGB palette the Gamma Factor and grayscale calibration affect the results Calibration Procedure Test patterns are required for each measurement in this procedure The Radiance processor can generate all the necessary test patterns internally Using the Radiance internal test patter
6. cal Tip for setup instructions 3 The Radiance HDMI output type is set by default to RGB To change the output type press MENU gt Output gt Style gt Style gt HDMI Format gt Type There are selections for RGB YCbCr 444 and YCbCr 422 Lumagen recommends setting the output of the Radiance to YCbCr 422 for displays that support this mode 4 Save the new settings by pressing Menu Save OK OK OK 5 Set the display controls to factory defaults or neutral including black level brightness white level contrast sharpness and all color settings Turn off all the special display features that are intended to enhance the image such as noise reduction and edge enhancement Lumagen recommends using the controls in the Radiance for these features 2013 Lumagen Inc 3 Radiance Tech Tip 2 Rev 5 10 11 During calibration disable Dynamic Iris and Dynamic Contrast on the display if your display has these features Lumagen recommends leaving these off because they can have a negative impact on the overall image quality for real images After each calibration step you may wish to view real images to double check the calibration up to that point The Radiance must be in Service Mode Press MENU 0910 to perform the following calibrations Wote Calibration Software selects service mode automatically You may wish measure the Pre calibration values You can do this manually or automatically b
7. control is programmable at the corners of a linear Gamma RGB color cube Red Green Blue Cyan Magenta Yellow and Black for a total of 8 points It uses a 3D RGB color palette to allow both the primary and secondary color points to be independently calibrated for x y and Y values The 3D palette works in linear RGB color space The Radiance processes the incoming values to remove the Gamma pre emphasis Next the 3D RGB palette processes the image Finally the Gamma pre emphasis is reapplied to the video Note The black corner of the cube Is adjusted in the Grayscale calibration menu The other seven corners of the cube are adjusted in the CMS menu Radiance 125 729 point CMS SW Rev 091612 081413 and later The 3D gamut can be set to use 8 125 or 729 points only 8 125 are available for Radiance Mini XS XD XE A new color cube is used for both the 8 125 and 729 point calibration which is located inside the older linear Gamma RGB color cube Note that 0 is 16 255 and 100 is 235 255 The 125 point color gamut consists of a 5x5x5 array of points The 729 point size available on 20XX or later units is a 9x9x9 array The calibration points are located at 2 2 Gamma reference points 0 25 50 75 and 100 video levels for Red Green and Blue in all combinations or 0 12 5 25 37 5 50 62 5 75 87 5 100 for 729 point gamuts The Radiance processes the incoming values to remove the Gamma pre emphasis Next the 3D RGB p
8. nce calculator which is available on the Lumagen website http www lumagen com testindex php module utilities This calculator will output the expected x y and Y values for each primary and secondary point Calibration software can take these readings and calculate expected results automatically The luminance Y value for white may need to be reduced in order to have enough luminance in the primary and secondary colors This is a common failing in displays and projectors which is easily overcome using the Radiance Between the primary and secondary colors find the color with a Y which has a measured value that is the smallest percentage of the calculated value If this is above the calculated value there is no need to adjust the white point Y value Otherwise reduce the Y of white by the same percentage as the percentage the selected primary secondary color is below its calculated Y value MENU gt Output gt CMS s gt CMS gt Color Gamut While performing this step make sure to control the R G B balance to maintain the correct x y point for white At this point you can work to get the x y and Y values for each primary and secondary color to match the targets by editing the RGB color values Start with the points farthest from their target values This step can be done using calibration software to simplify the task There is a bit of art in this adjustment since the RGB calibration color space does not align
9. ns will provide the most accurate calibration results The following procedure will work for an 8 point 125 or 729 point CMS calibration Due to the greater number of points we recommend using automatic calibration software for a 125 729 point calibration ChromaPure SpectraCal CalMAN and Light Illusion have developed automated calibration software for the Radiance Note When using automated calibration software you should follow their workflow for the calibration The Radiance has many setup and calibration features Refer to the User Manua and Technical Tips posted on our website for details about the proper setup of your theater It is important to do the calibration steps in the following sequence 1 Install the Radiance For the XS XE 20XX connect an audio receiver or Pre Pro to HMDI Output 1 and a video display to HDMI Output 2 By default the Radiance automatically selects video output resolution and rate and reports supported audio modes as read from the device on Output 1 or for the RadianceMini the only output back to the sources If you need to change the output modes or audio modes see the Radiance User Manua for your unit 2 The Radiance output aspect ratio which should match the display is set by default to 16 9 1 78 To change the output aspect ratio press MENU gt Output gt Styles gt Style gt Aspect Ratio Wote For a projector that has an anamorphic lens see the appropriate Radiance Techni
10. times as low as 1 8 to 1 9 This is often done to mask dark area noise issues in the display When this display flaw exists calibrating for a correct Gamma of 2 2 is likely to expose this dark area noise In this case you need to make a trade off between correct Gamma and minimizing the visibility of the dark area noise The Gray Window pattern can be set to small normal or full screen window A small window pattern works best on plasmas and other displays where the result varies depending on the area covered by the window pattern due to the display s power supply limitations It is best to use the Radiance Window 1 or Window 2 patterns Note Press 4 to select between Window 1 medium Window 2 small Window 3 large and external test patterns Press 8 to show hide the menu Window 1 Window 2 Window 3 Measure the Gamma curve of the display in either 5 or 10 increments and calculate the average Gamma Use the Lumagen Gamma Factor to approximate the desired display Gamma Set Gamma Factor Desired_Gamma Measured_Gamma In order to support whiter than white and blacker than black in the Gamma Factor Lumagen includes the entire range of values i e 0 to 4095 with 12 bit pixel depth where black 256 in the Gamma curve This means that changes to the Radiance Gamma Factor also change the black and white levels Note black and white levels should be recalibrated after changing the Radiance Gamma Fac
11. too bright If black is too bright the image will look softer than it should o o o S o o o 3 9 o I gE 2 2 1 i o S gt o o e I Contrast 1 Contrast 2 Black Ramp For white level the 96 and 104 vertical bars should be differentiable from the 100 background and the 104 bar should not be crushed which may show as a color change verses the 100 background and 96 bar Use the Radiance Contrast 1 and White Ramp patterns Ramp 4 to 4 Contrast 1 White Ramp Check the default Gamma curve of the display Specifications call for a Gamma of 2 2 however many professionals prefer a Gamma of 2 4 You can use either the 2013 Lumagen Inc 4 Radiance Tech Tip 2 Rev 5 display s Gamma control function or the Radiance s Gamma Factor control to do an initial coarse calibration of Gamma Depending on the display it may be necessary to later use the Radiance s 11 or 21 point parametric Grayscale Gamma function to correct for non linearity in the display Gamma These steps must be performed before the gamut calibration The Gamma Factor control MENU gt Output gt CMS s gt CMS gt Gamma Factor allows either internal patterns or an external pattern generator to be used Many digital flat panel displays intentionally come with a low measured Gamma some
12. tor 12 If the display has a grayscale calibration feature you may choose to adjust the 100 point grayscale using the display s calibration control In this case use the Lumagen 100 gray Window 1 or Window 2 patterns Window 1 Window 2 Window 3 13 Calibrate grayscale and Gamma using the Radiance 11 or 21 point parametric grayscale Gamma calibration MENU gt Output gt CMS s gt CMS gt Gray Gamma If you have an auto calibration program this can be done automatically Vote Once any gamut entry is changed do not change any grayscale entries or the Gamma Factor 2013 Lumagen Inc 5 Radiance Tech Tip 2 Rev 5 14 15 16 17 18 19 20 21 22 Recheck the black and white level of the display and adjust if needed Note This may affect your Grayscale Gamma calibration and if so you might need to redo those steps while leaving the black and white levels as adjusted in this step The Radiance Color Gamut control can only bring display color points toward the center i e toward the white point of the CIE diagram That is the Color Gamut control can t drive the display s color beyond its physical capability It is important to note that many displays have a color temperature selection e g default warm natural etc These selections generally affect the primary and secondary x y points Before starting the color gamut calibration when possible select a color mode for which all
13. y using calibration software This can provide a good comparison of pre and post calibration results Set the black level brightness and white level contrast using the controls on the display If the display controls are too coarse or lack the proper range you can use the Radiance output configuration black level and white level to adjust the black or white output level MENU gt Output gt Styles gt style gt HDMI Format gt Type When black is properly calibrated a 4 bar cannot be differentiated from a 0 black background and a 4 bar can be barely seen against a 0 black background In fact in a well designed display projector when properly calibrated even a 1 bar cannot be differentiated from a 0 black background and 1 bar can while viewing the Black Ramp contrast test pattern from the Radiance Note Use the Radiance Reference test patterns for setting the display controls It is important to note that many displays will have different black levels depending on the average picture level APL of the image The Radiance Contrast 1 pattern with a 4 and a 4 vertical bar on the sides is a high APL image It is better to use the low APL Radiance Contrast 2 pattern which has 4 to 4 vertical bars in 1 steps in the center and the Black Ramp pattern to insure that dark scenes have a correct black level setting Vote Setting black slightly too dark is better than setting black slightly
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