2 - itd lecture intro to display measurements i 2010.ppt

8/11/2019 2 - ITD Lecture Intro to Display Measurements I 2010.ppt

1/20

Dr. Steve Clark

[email protected]

With thanks to Dr Cranton and Dr Tsakonas

Interface Technology & Design2010/11

Module ITEC 20121

Lecture 2Introduction to Display Measurement


2/20

Warning!

This lecture contains a lot of technicalinformation on light measurements and

units that requires some familiarisation tounderstand fully (and information beyondthe syllabus covered). We use thelaboratories to help develop the aspects

that are important to the module.


3/20

The challenge for visual interface technology is to provide a display that willenable us to see images with a quality that is limited by the capabilities of

human vision rather than the characteristics of the display.

Approximate Specification of The Human Visual System

Luminance detection 3 x 10 -6 cdm -2 - 3 x 10 6 cdm -2

Spectral response 390 nm - 760 nm (the visible spectrum)

Colour Acuity Variable (see later lectures, thousands typical)

Normal Visual Acuity 1 arcmin ( 1/60 )

Binocular Field of View 200 H, 120 V (160 H, 120 V monocular)

Fusion Frequency below 60 Hz (variable)Contrast Sensitivity 1% (variable)

Al l dependent upo n env i ronment , backgrou nd luminance , s t ate o f adap tat ion


4/20


5/20

PLASMA EL FED VFD LED

Emissive displays

ECD DLP EPD

Passive displays

CRTs Flat Panel Displays

Displays market

LCD

We will measure theDisplays shown in red

Measurements:

Luminance meter (cdm -2)

Spectrometer (uncalibrated spectrum)

Chromaticity meter (x, y)


6/20

Brightness, Luminance and Radiance

A display will have a particular radiance which will give an average visual response of aparticular luminance, but the perceived brightness depends on several other factorsincluding the individual differences from the average, the environment, the adaptationstate of the eye, contrast, colour and the interpretation of the brain. So, brightness is notproportional to luminance and in an optical illusion can even contradict luminance.

Brightness is a subjective quantity describing appearance.

Radiance is a repeatable objective absolute physical measurement.

Luminance is a repeatable objective measurement relating to vision.


7/20

Radiometric and Photometric UnitsEnergy is propagated as electromagnetic radiation termed - radiant energy thefundamental unit of which is Radiant Flux , measured in Watts , W. The radiant flux isthe energy flowing through a surface per second . The system of units based on this

are known as Radiometric Units

Photometric Units relate radiant units to the average visual response. Hence, thephotometric equivalent of Radiant Flux is Luminous Flux which is a measure of thetotal visual output of a light source, measured in Lumens, lm (care: not 1 metre!)

[This looks complex but just takesinto account the contributions ofall wavelengths by integration]

The conversion factor for the peak of this response is 683 is for normal photopic visionHence 1 lm = 1/683 W at the peak response wavelength of 555nm (or 1 W =683 lm)( The scotopic conversion factor = 1745 )

( ) F F

F F

=

= = =

683 350nm

760nm V e d

e V

l

l

l

Luminous Flux Radiant Flux Relative Luminous Efficiency ( )


8/20

Brightness is defined formally as the attribute of a visual sensationaccording to which an area appears to emit more or less light.

Luminance (Cdm -2 or lmsr -1m -2) is the radiance from an area weighted bythe spectral sensitivity associated with the (population average) sensationof vision for the spectrum observed. Light is emitted from an area withvarying angular dependence (usually lower levels at higher viewing angles).

Luminance is more useful measure for displays than Luminous Flux as it linksto a typical display area emitter with angular dependence. It is proportional to itsphotometric equivalent unit, Radiance (Wsr -1m -2), for a visual stimulus.

Steradian (sr) is the unit of solid angle. For a flat surface emitterthe light output will be into to a hemispherical solid angle whichis by definition 2 steradians (a sphere is 4 ).

Some aspects of colour and colour vision


9/20


10/20

Some Typical Luminances (cdm-2)

Surface of the sun 2.0 x 10 9 Fluorescent lamp 6000CRT monitor (max) 310

White paper (bright sunlight) 25000White paper (bright moonlight) 0.03

Some Typical Display Luminances

TVs (max) 300 500Monitors (max) 250 - 350TFT LCD notebook (max) 70

Photometric Units


11/20

Contrast Measurements (see labs)

Contrast Ratio is typically defined as on-state luminance divided by off-state luminance:

CR = L on / L offwhere the L terms refer to pixel luminance for on and off screen inputs (white and black)

Luminance Contrast is usually defined as: CL = L on - L off / L off

Luminance contrast is arguably the most important parameter to define the visual quality ofan interface. We will use this parameter to compare display technologies in the lab.

It is important to specify ambient conditions and the angle used for measurement.

Peak contrast is often quoted very misleading (but helps sell displays?), it is better to usea standard test pattern (as we do in the lab).


12/20

Luminance Measurements

Uses a specially designed detector which

includes filters that ensure the spectralresponse matches that of the eye. Can often select measurement units (we

will select cdm -2 ).


13/20

Chromaticity Measurements

This relies the CIE 1931 Colour Model (see later lectures). Uses a Luminance meter with special additional filters that

match the spectral response of the Chrominance readingbeing taken.

Measure values of X, Y (or more usually the normalisedvalues x, y) for a uniformly coloured area of a screen.

Calculate z if required (from x + y + z = 1). Plot the x, y coordinate for the screen area on the CIE

chart. Y is set to be directly equivalent to luminance.


14/20

CIE Chromaticity Triangle

Measurement of x, y, z coordinates is performed using chromaticitymeters (luminance meters with X, Y, Z filters)

CIE x - y plot

x = red y = green (z= blue)

Normalised solower case

x + y + z =1

White

.33x +.33y+.33z


15/20

Spectral response of rods and cones (normalised)


16/20

The CIE 1931 Tristimulus Spectra

The tristimulus valuefor red, X, is derivedby measuring the

output of a light source using an X filter(with response x(l ) ).Repeat for Y and Z.
http://en.wikipedia.org/wiki/Image:CIE1931_XYZCMF.pnghttp://en.wikipedia.org/wiki/Image:CIE1931_XYZCMF.png


17/20

Construction of the RGB Display Colour Gamut

Measure values of x, y for all three Display primaries: RedGreen and Blue.

Plot the three x, y coordinates on the CIE chart (see later

for more details on the CIE chart) Draw the triangle formed by the three coordinates. The only saturated colours producible by that display now

lie within the triangle formed by the true x, y coordinatesfor the Red Green and Blue display primaries. The onlycolours will include those saturated colours mixed withgreys.

Screen reflections will distort x, y coordinates measured ina lab for RGB (normally moving towards white).


18/20

CIE Chromaticity Triangle

Measurement of x, y, z coordinates is performed using chromaticity meters

Measure x and yvalues for eachCRT primaryand plot onCIE chart

x + y + z =1

White.33x +.33y+.33z

x

y

CRT Primaries

CRT Colour Gamut


19/20

Some key points for the Lab

Luminance measurements give a calibrated measure of(screen) output in cdm -2

Chrominance measurements give a calibrated measure ofaverage perceived colour as Y, x, y where x and y are

coordinates on the CIE colour chart. (no units) and Y isluminance. Luminance Contrast measurements give a calibrated

measure of real screen contrast values (a ratio, so nounits).

Measurements are taken under approximately normal labconditions to model normal screen use (deliberately not ina dark room).


20/20

2 - itd lecture intro to display measurements i 2010.ppt

Documents