The ND Filter Debate: Light Illusion vs. Portrait Displays

Clashing Philosophies

As HDR displays push peak brightness beyond 4,000 nits, the calibration industry faces a pressing challenge: how to accurately measure extreme luminance ranges without compromising color accuracy. Light Illusion, led by CEO Steve Shaw, and Portrait Displays hold diametrically opposed views on using ND filters to extend probe capabilities. Shaw champions ND filters paired with advanced software correction as a cost-effective solution, claiming it preserves accuracy across the entire dynamic range. Portrait Displays counters that inherent optical compromises of ND filters undermine calibration integrity, introducing spectral shifts and measurement discontinuities that can corrupt professional workflows. Can calibrators rely on this band-aid approach for cheap probes to measure HDR displays?  

Light Illusion’s Position: Software as Savior?

Steve Shaw frames ColourSpace’s ND filter integration as a breakthrough that defies conventional sensor limitations, emphasizing accessibility and precision:  

“Using the inherent capabilities within ColourSpace, the Dynamic Range of ANY probe can be extended while keeping the probe’s inherent low-light capability, and maintaining the same accuracy throughout the extended dynamic range.”

Shaw targets lower-cost probes like the X-Rite i1D3, suggesting that ColourSpace’s algorithms theoretically compensate for hardware constraints:

“Normally, any approach to extend peak brightness capability reduces low-light sensitivity… Using ColourSpace, an ND filter increases peak brightness readings with no impact on lower brightness capabilities.”

The method involves replacing a probe’s diffuser with an ND filter and using ColourSpace’s engine to rescale measurements. Light Illusion claims this enables sub-$500 probes to measure ≥ 5,000 nits — a domain traditionally requiring prohibitively expensive reference-grade spectroradiometers. Critics, however, note this physically alters the probe’s optical characteristics.

Shaw’s confrontational style – which led to his permanent banishment from industry forums like Lift Gamma Gain for repeated toxic behavior – resurfaced in his dismissal of Portrait Displays’ white paper as ‘dodgy marketing.’ This combative approach mirrors past incidents where Shaw hijacked discussions to promote his software, driving experts like color scientist Dado Valentic to lament that forums were ‘hijacked by that lunatic… [so] most smart people… left.’ While Shaw’s technical arguments about ND filters merit scrutiny, this pattern underscores how personal clashes often distract from substantive debates about calibration accuracy. As one veteran colorist revealed, Shaw’s forum conduct epitomized the ‘egotistical, closed-off’ voices that devalue online discourse-contrasting sharply with the ‘curious, humble’ professionals advancing the field.

Portrait Displays’ Rebuttal: The Perils of Optical Compromises

“Many calibration pros have tried it: your meter is out of range, so you pop an ND (Neutral Density) filter in front to cut the luminance. Smart solution—or risky shortcut? While it may sound like a clever fix, adding external ND filters can introduce new variables that undermine the accuracy of your calibration workflow. In fact, it often causes more harm than good.” – Portrait Displays

Portrait Displays’ white paper methodically dismantles the ND filter approach, highlighting three fundamental flaws:  

1.  Spectral Non-Neutrality:

ND filters introduce persistent color casts that corrupt chromaticity measurements.  

2.  Measurement Discontinuities:

Switching between filtered/unfiltered measurements creates severe, uncorrectable breaks in EOTF curves.  

3.  Uncontrollable Variables:

Filter performance drifts due to aging, contamination, temperature sensitivity, and angular dependence — issues software cannot resolve. Mounting inconsistencies further scatter light unpredictably.  

Practical Implications for Professionals

The debate crystallizes key trade-offs for calibration workflows:  

– Cost vs. Precision:

Light Illusion’s method democratizes HDR calibration, but Portrait Displays argues this risks artifacts—like skewed color volumes—that may only surface in downstream color grading.  

– Workflow Efficiency: 

Shaw’s “set-and-forget” ND solution purportedly simplifies measurements. Yet Portrait Displays’ data shows knitting filtered/unfiltered data requires manual patch selection and transition smoothing, significantly increasing calibration time.

Pragmatism or Purity?

For budget-conscious studios calibrating consumer HDR displays, Light Illusion’s approach offers a temporary stopgap. For reference-grade facilities mastering cinematic or broadcast content, Portrait Displays’ caution is critical: ND-induced errors consistently exceed SMPTE/ITU tolerances for color deviation (<1 ΔE) and luminance consistency (<2%) by orders of magnitude.

Until probe technology advances, professionals face a clear divide: ND-enabled probes suffice for YouTube HDR workflows but fundamentally compromise accuracy where color integrity is paramount.