EIA TEP105-17-1990

MTFs FOR MONOCHROME CRTs

Background

In most cases, the interface between humans and machines is a
visual display.

The sharpness and readability of raster-scanned Cathode-Ray Tube
(CRT) displays conventionally has been determined by how far the
vemcal raster height could be reduced while still displaying
individual scan lines (Shrinking Raster Line Width Measurement).
This method yields a rough "order-of-magnitude" estimate of the
visible raster-line capacity of a display device.

As user demands for display readability increase, the
subjectivity and over-simplification of this technique limits its
practical utility for precise engineering specifications. A more
definitive method of measuring the true beam spread is needed to
describe the complete display performance. The measurement of line
width by a scanning photometer removes a great deal of the
subjectivity from this type of measurement. in this method, a
measurement is made of the light distribution across the width of a
single raster-he. This measurement reveals the true profile of
light intensity across a raster-line in an objective manner. The
line profile, in tum, usually is designated by its full-width at
50% of its peak light intensity or luminance.

Moreover, continual user demands for better image quality
through increased pixel density has underscored the need for more
descriptive methods of assessing the total imaging capability of a
CRT display. A quantification of the accuracy achieved in
reproducing the input signal on the viewer's screen is the ultimate
goal. The preferred specification to describe this performance
characteristic of a CRT display is the Modulation Transfer Function
(MTF).

The MTF takes into account not only the raster-line width at one
specific percentage of peak intensity, but also accounts for the
entire line profile shape. It can be shown analytically that the
shape of the raster-line profile as well as the beam width are
evaluated in the MTF. Phosphor screen spreading, reflections from
the phosphor and optical coatings, as well as saturation effects of
the screen also affect to the MTF.

An additional advantage of the MTF measurement is that other
elements of the display system may be incorporated into the
evaluation, and thus, provide a composite MTF specification for the
total display system. The MTF usually is presented as a curve
representing modulation transfer across the spatial frequency
bandpass of the display device or system.

One method to evaluate the MTF is to drive a CRT grid or cathode
with a sine-wave signal. Synchronized with an appropriate
raster-scan, typically 525 lines, bright-to-dark sine-wave bars are
generated on the display screen. Reference conditions can be
selected to assure full-on and full-off performance of the display.
The bright and dark bars are scanned with a slit-aperture
photometer such as that described in EIA Test Method 105-7.

Statement of Purpose

The purpose of this test method is to provide a procedure to
compute the MTF of monochrome CRTs in order to quantify the
discemability and quality of displayed information. While the
mathematical concepts of the MTF are complex, this test method
provides a software package which aiiows users to easily convert
spot profile data into MTF data., There is no need for mathematical
calculations by the user. The software package is designed for use
on any computer system with a standard FORTRAN-77 compiler, and a
second version of the software for an IBM-PC (or lûû%-compatibles)
personal computer provides graphical presentations of the spot
profile and associated MTF data.