Below are color fidelity measurements of the various X61 screens fitted with LED strips from several different vendors. I made these measurements before I began building my own kits and strips, mainly as a way of quantifying just how dire the color reproduction problem really was.

I also included a number of strips I made by hand using general-purpose white lighting LEDs as some early tests (which made it clear such LEDs were entirely unsuitable), as well as a few other LCD screens for comparison purposes.

Be aware that most vendors change the LEDs they use almost constantly, but this should still give some idea as to the kind of color fidelity roulette one can expect from a random LED conversion kit.

Notes: Measurements taken from a batch of apparently unused, factory-sealed HV121P01-100 screens with front cover 'glass' in place.


How to read the charts

CIE colorspace

Short version: The closer the red triangle and + are to the white triangle and D65 circle, the better.

Longer version: The CIE 1931 color chart on the top left is a plot of how close the screen/backlight combination comes to accurate reproduction of basic colors. There are several versions of the CIE colorspace; I've chosen the oldest and most familiar. They all impart the same information, they just scale the color areas differently.

The large, curved, vaguely horseshoe-shaped area represents all the colors humans can see.

Inside the CIE horseshoe, the highlighted white triangle marked 'sRGB color gamut' indicates the default standard computer monitor colorspace, that is, all the colors a standard computer monitor should be able to display. The screens we're measuring here should be judged against sRGB, so the white triangle represents perfection. Near the center of the triangle, the circle marked 'D65' indicates the position of ideal white (a color temperature of 6504K on the daylight locus. The daylight locus sits slightly above the blackbody locus marked by the black curve).

The red triangle indicates the actual colors produced by the display being measured. A red triangle smaller than sRGB means colors look washed out in comparison. A red triangle larger than sRGB means colors look oversaturated. Where the triangle corners don't line up, that color is shifted in hue. For example, it's common for low-power displays of this era to shift deep blue toward cyan in order to increase apparent brightness.

The red '+' indicates the actual measured white color produced by the display. The closer the plus is to the center of the D65 circle, the closer the display's whitepoint is to correct.

Color bars

Short version: Given the color on the left side, the display actually shows the color on the right side. The closer the colors on the left and right match, the better.

Longer version: The color bars illustrate the relative color differences between an ideal sRGB display (the reference color on the left), and the same color as it would be displayed on the measured monitor (the displayed color on the right). Of course, very few displays perfectly match sRGB, so even the absolute reference colors are going to be displayed differently on different monitors. However, the relative color difference will still be approximately correct so long as the display in use is anywhere close to sRGBish behavior.

Each color bar also contains a few numerical measurements. The white bar shows the correlated color temperature, brightness of the display in nits, and the CIEDE2000 perceptual error between the measured whitepoint and the closest point on the daylight locus. The black bar shows the measured blackpoint brightness and contrast ratio of the display. The color bars each show the relative measured saturation (actually 'perceived colorfulness', but close enough) of red, green and blue as compared to the reference in the CIELCH colorspace, as well as the CIEDE2000 perceptual error between the reference and measured primaries taking the measured whitepoint into account. Ideal saturation is 100%, and ideal CIEDE2000 error is 0.0 (less than ~3 is usually considered imperceptible).

Raw spectral power density

Short version: Don't worry about it.

Longer version: The SPD (spectral power density) is the raw power-per-spectral bin data collected by the spectrometer. The software makes separate readings for red, green, blue, white and black. This information is then used to compute the various color metrics.

The SPD plot shows this data in raw form, normalized so the highest peak always fits on the graph. In this form, the data doesn't say much [directly] about final color rendering, but the curves can tell us both about the backlight technology in use (CCFL, white LED and RGB LED backlights will all look significantly different on such a graph) as well as the color filters used in the LCD panel.

Measurement details

These measurements were all taken using a UPRTek MK350N handheld spectrometer that returns absolute mW/m²/nm spectral irradiance data in 1nm bins from 360nm to 760nm. This device is designed for relatively high-intensity measurements and features a wide-angle cosine-corrected sensor meant for ambient lighting use. Monitor measurement, on the other hand, requires low light sensitivity and a narrow field of view (not to mention conversion to nits).

I designed a measurement harness for the spectrometer with narrow-field correction optics and a large aperture to give it a roughly 10 degree field of view and boost light gathering by about a factor of 15x. The optics plus long exposures and noise profiling software gives the device a minimum useful sensitivity/accuracy of about .005nits. Reproduceability was well within than the natural brightness variability of the backlight hardware.

Above: These measurements were made by fitting custom optics to an MK350N spectrometer (using only the highest-precision laboratory grade foamcore and hot glue), then fitting the spectrometer and optics to a mounting baffle that enveloped the LCD panel.

Because of the very narrow field of view, the measurement figures are a bit different compared to consumer grade monitor calibration hardware. Contrast and brightness numbers especially may be somewhat higher than you'd measure with, e.g., a Spyder colorimeter. The narrow field of view is blind to off-angle glow and bleed and measures the emitted light in its strongest direction, so it gives near-optimal 'dead on center, straight-ahead' scores.

Notes and observations

We can draw several conclusions from these measurements.

None of these panels are high-saturation under the best of conditions.

Even high quality general-purpose lighting LEDs (e.g., the Osram, Cree, Luxeon, Toshiba, and Hebei LEDs above) have inappropriate spectral output for backlights. 6500K LEDs do not result in a 6500K panel whitepoint. The LCD panels themselves filter the light nonuniformly, and the spectrally yellow-green heavy output of high efficiency lighting LEDs is selectively overemphasized.

LED backlight replacements using white LEDs based on a blue pump + yellow phosphor (ie, nearly every white LED produced today) shift the saturation balance significantly. The overall impression can still be acceptable, though with a noticable loss of saturation and a shift of reds and greens toward yellow.

Specialty LEDs designed for backlight use (eg, LEDs that use blue to pump separate red and green phosphors with little yellow overlap) can avoid this problem. They can, in fact, improve saturation and color rendering over the original CCFL.

Badly matched LEDs can skew the color output pretty hard. And, unfortunately, the highest-efficiency LEDs tend to have the worst color (as they lean harder on the yellow/green portion of the spectrum for higher apparent output).

Whatever the LED used, they're between somewhat and substantially brighter than a CCFL.

Lastly, different screens looked best with different LEDs. The AFFS screens especially require LEDs with lower-than-usual green output, and as little as possible yellow. Most of the LED conversion kits being sold on the web appear to be more appropriate for TN screens. Few look good when used with AFFS.


—Monty ( May 22, 2015