Characterization of a LCD display

Tanmay Mondal University of Eastern FinlandYousuf Hemani PhotonicsMohammad Al Lakki

What characterization of a display means:

A process of modelling the display and to measure the properties to get the required image. The goal is to establish a relationship between digital input values (RGB) and tristimulus values (XYZ).

Colour is produced by giving a set of input values to the device which are the RGB colour coordinates.

The colour characterization of a display means finding the relationship between the device dependent colour coordinates (RGB values) and the device independent colour coordinates (XYZ tristimulus values or CIELAB coordinates) which are related to human visual system. This relationship is defined experimentally by the colour characterization measurements.

Backward implementation of the same process where we give the XYZ values and to reproduce the desired colour on the display device.

RGB colour coordinate:

The simplest coordinate system is RGB coordinate where R, G, B represents the basis vectors of colour coordinate system.

R, G, B represents the amount of red, green and blue colour.

Addition of three colours produces a new colour and the new colour depends on the amount of the three principle colour that has been mixed.

So, RGB system is physically realizable system.

XYZ colour space:

XYZ is another coordinate system where X, Y, Z are called the tristimulus values.

They produce another set of basis vectors and the coordinate system defined by them is different than RGB coordinate. This XYZ coordinate system is completely virtual and mathematical.

XYZ depends on human eye sensitivity. Every linear combination of XYZ corresponds to unique combination of RGB, means there is one-to-one correspondence between XYZ and RGB.

LCD display:

• A basic LCD has two glass plates with transparent electrodes inside their surfaces and a liquid crystal material sandwiched between them.

• When a known voltage is applied to the substance, it gets untwisted in varying degrees according to our requirement.

• If the polarizer at the output end is parallel to the one at the input end, it is called normally black mode and if the output polarizer is perpendicular to the input polarizer it is called normally white mode.

Objective:

Two models were used to predict the DAC values that are needed to reproduce a colour on a monitor from a defined XYZ tristimulus values of a sample colour. (Macbeth colorchecker)

Linear model:

spectral radiance of red vs. wavelength for seventeen different values of dr (starting form 1/17 to 17/17), while dg and db were kept fixed at

0.

Linear model:

CIE Colour Matching Functions

Linear model: Matrix relation between XYZ and RGB coordinates

= +

The , and these matrix elements are tristimulus values and can be calculated. The values can be found out by Spectroradiometer if one puts the DAC (dr, dg, db) values to (0 0 0) as an input while measuring the spectral radiance.

Linear model:

R,G,B vs the DAC values for each channel

Masking model:

• The order of magnitude of dr, dg and db is known.

• As db has the smallest magnitude, so this much amount will be replaced by Grey.

• Green has the second highest magnitude. So it will be replaced by yellow at value dg minus yellow at value db.

• Finally Red has the highest magnitude. Thus it will be replaced by red at value dr minus red at value dg.

RGB values to masked values conversion

Magnitude of the “projected XYZ vector of the primary and secondary channels on the first principal component” at different DAC values, the C values.

Equipment and experiments:

Initially we used the laptop screen for Dell Latitude D620 for our measurements. But due to several issues with the brightness and the displaying of colour blue we had to change the display.

The final measurements have been done with a Dell monitor display. We have converted the final results to CIELAB colour system to find the colour difference to check the accuracy of both the models using Matlab.

Characterization results:

For the masking model the average colour difference was about 1.8 which is an improvement over the linear model.

For the LUT model the colour difference is between 2 and 4 for most of the patches in the tested sample (average E* is 3.39).

Results of colour reproduction:

To check the masking model, random colours were generated on the display. Their spectral radiance was measured and the DAC values were obtained. After that the DAC values were used to generate the colours again to check whether we will obtain a match.

Challenges:

Viewing angle (we could have obtained better match if the Spectroradiometer was adjusted at the optimum position, i.e. to resemble the one used when the data was collected).

Human error (a mishap could have seeped into the code, we had no time for second trials and double checking).

The display is producing slightly different colours at different times of operation.

The Imperfection of our model. Interpolation data based on 17 measurements only.

Thank you