Ambient Light Effects in Pupillometry

Quantitative Pupillometry Eliminates Measurement Errors

Pupillary assessment remains one of the most fundamental components of neurological evaluation in critically ill patients. Since the pioneering work of Plum and Posner in the 1960s, clinicians have relied on pupil size and reactivity to predict lesion localization, determine surgical eligibility, and monitor neurological status. The emergence of quantitative pupillometry promised to eliminate the inter-rater variability that plagues subjective pupillary examination. Automated devices have been shown to more reliably record pupil characteristics and identify changes earlier than manual measurement alone.

Yet recent research demonstrates that even these automated infrared devices are not immune to the confounding effects of environmental lighting, creating systematic measurement bias that directly affects patient care decisions.

Environmental Factors Still Create Systematic Measurement Bias

Research by Charlene Ong (Boston Medical Center, USA) and colleagues provides compelling evidence of ambient light's impact on quantitative pupillometry. Their investigation involved seven healthy volunteers and seven critically ill patients, with pupillary responses tested using infrared pupillometers under carefully standardized lighting conditions.

The researchers established bright conditions using "overhead LED lighting in a room with ample natural light" at approximately 180-200 cd/m², while dark conditions consisted of "a windowless room with no overhead light source" (<1 cd/m²). Each participant underwent ten measurements in both lighting environments, yielding 280 total observations.

Results Show Critical Patients Shifting Between Normal and Abnormal Pupil Reactivity

The findings revealed significant differences between lighting conditions. In healthy subjects, the median composite pupil reactivity score changed from 4.2 in bright conditions to 4.3 in dark conditions. However, the implications become more pronounced when examining critically ill patients, where median values shifted from 2.85 in bright conditions to 3.3 in dark conditions (Ong et al.).

This research demonstrated that ambient light affects pupillary measurements differently across patient populations, with critically ill patients showing more substantial variations than healthy controls. The study found that "ambient light levels impact pupil parameters in both healthy and critically ill subjects. Changes in NPi under different light conditions are small and more consistent in healthy subjects, but significantly differ in the critically ill" (Ong et al.).

Independent Prospective Study Validates The Findings

A 2025 prospective crossover study corroborated these findings in a larger cohort involving 20 adult ICU patients requiring invasive ventilation (Holmskär et al.). This validation work employed a more rigorous design that tested patients under bright conditions, then dark conditions, then bright conditions again, observing consistent patterns of ambient light influence.

The study utilized standardized lighting conditions achieving "an illuminance of 1 lx under DC [dark conditions] and 301 lx under BC [bright conditions]". Results showed that composite pupil reactivity scores were significantly lower in bright compared to dark conditions in both eyes, with 25% of participants experiencing score decreases of 0.6 or more when transitioning from dark to bright conditions.

Most concerning for clinical practice, researchers noted that several patients experienced multiple readings indicating non-reactive pupils in bright light, but normal reactivity in dark conditions. This finding suggests that ambient lighting conditions could lead to false-positive readings of absent pupillary reactivity.

These Lighting Effects Can Create False-Positive Non-Reactive Readings

The study concluded that ambient light significantly impacts pupil measurements and recommended that "practitioners should standardize lighting conditions to maximize measurement reliability". However, achieving such standardization in real-world clinical environments presents substantial challenges.

The Clinical Threshold System Makes Small Changes Diagnostically Significant

Automated pupillometry devices typically operate using composite scoring systems that range from 0 to 5, where values >3 are considered normal. These scores represent proprietary metrics based on comparison of measured pupillary responses against normative models of pupil reaction to light.

Understanding how ambient light affects these scales is crucial for clinical interpretation. Research indicates that while device manufacturers can claim consistent performance "regardless of the ambient conditions," the evidence clearly demonstrates that ambient light levels require standardisation for measurement independence.

The clinical threshold of 3.0 for normal reactivity becomes particularly important when considering these validation findings, where lighting changes could shift measurements across this critical boundary (Holmskär et al.).

The Problem Affects Every Major Measurement Parameter

The research reveals that ambient light most significantly impacts:

1. Baseline pupil size - directly affected by ambient lighting conditions
2. Constriction amplitude - the difference between resting and constricted pupil size
3. Constriction and dilation velocities - the speed of pupillary response
4. Composite reactivity scores - particularly in critically ill patients

Ong and colleagues used "multi-level linear regression to account for both inter- and intra-subject variability" and found that "NPi, resting pupil size, constricted pupil size, pupil size change, constriction velocity, and dilation velocity were significantly different depending on ambient light conditions" (Ong et al.).

Notably, latency (the time delay before pupil constriction begins) appears to be the least affected parameter across different lighting conditions in both studies (Ong et al., Holmskär et al.).

ICU Environments Make Consistent Lighting Nearly Impossible to Standardize

The clinical implications extend beyond individual measurements. Research emphasizes that "In patients in whom pupillary reactivity is being trended for signs of clinical deterioration, examiners should aim to recreate consistent ambient light levels" (Ong et al.). This requirement becomes particularly challenging in ICU environments where lighting conditions frequently change due to:

• Day/night cycles affecting natural light
• Varying overhead lighting during procedures
• Different lighting preferences among staff
• Emergency situations requiring altered illumination

This validation work specifically tested conditions that "fell within the range of conditions that nurses took pupil measurements in as standard of care," demonstrating that typical ICU lighting variations can significantly impact measurements (Holmskär et al.).

The Evidence Shows Ambient Light Can Directly Impact Patient Care Decisions

The impact of ambient light on pupillometry measurements represents a significant challenge in neurological monitoring that requires immediate attention. Research by Ong and validation work by Holmskär clearly demonstrate that environmental lighting conditions can substantially influence pupillary assessments, potentially affecting critical clinical decisions (Ong et al., Holmskär et al.).

For neurosurgeons and intensive care teams, this evidence underscores the importance of standardizing measurement conditions and accounting for the confounding effects of ambient light when interpreting pupillometry results. The validation study's finding that patients could show non-reactive readings in bright light while maintaining normal reactivity in dark conditions represents a particular concern for clinical practice (Holmskär et al.).

Phone-based Computational Pupillometry Emerges as a Solution to Environmental Limitations

While traditional infrared pupillometers struggle with ambient light variability, recent technological advances have introduced smartphone-based solution that incorporate automated ambient light correction algorithms. Solvemed’s PuRe Pupillometer represents a breakthrough in addressing these fundamental measurement challenges through machine learning approaches to ambient light correction.

A comprehensive solution using smartphone-based pupillometry has been developed by Solvemed, incorporating machine learning algorithms specifically designed to correct for ambient light effects (Bogucki et al.). Their investigation involved testing across ambient conditions ranging from complete darkness (<5 lx) to bright lighting (>10,000 lx), encompassing virtually all clinical environments.

The study assessed the sensitivity of seven PLR parameters to differences in ambient light using a smartphone-based pupillometer. The researchers found that lighting most strongly affected initial pupil size, constriction amplitude, and velocity. They developed nonlinear models to find correction functions that maximally stabilized PLR parameters across different ambient light levels.

The Pupil Reactivity Score Provides Lighting-Invariant Measurements

The Pupil Reactivity (PuRe) score represents a significant advancement in pupillometry, quantifying pupil reactivity on a scale of 0-5 (0 = non-reactive pupil; 0-3 = abnormal/"sluggish" response; 3-5 = normal/brisk response) (Bogucki et al.). This score demonstrated 100% accuracy in discriminating unreactive pupils while remaining stable under changes in ambient illumination across four orders of magnitude.

The lighting-corrected parameters were combined using machine learning optimization to produce this scalar value. The score discriminated unreactive pupils with complete accuracy and demonstrated remarkable stability across varying lighting conditions (Bogucki et al.).

Conclusion

The impact of ambient light on pupillometry measurements represents a fundamental challenge that has been inadequately addressed in clinical practice. Research studies by Ong and Holmskär clearly demonstrate that environmental lighting conditions can substantially influence pupillary assessments, potentially affecting critical clinical decisions (Ong et al., Holmskär et al.).

For neurosurgeons and intensive care teams, this evidence underscores the importance of standardizing measurement conditions and accounting for the confounding effects of ambient light when interpreting pupillometry results. The research consistently recommends that "practitioners should standardize lighting conditions to maximize measurement reliability" (Ong et al., Holmskär et al.). However, the practical challenges of ICU environments make consistent lighting control difficult to achieve in real-world clinical settings.

Emerging smartphone-based computational pupillometry with automated lighting correction algorithms offers a promising technological solution to these fundamental limitations (Bogucki et al.). These systems demonstrate the potential to maintain measurement accuracy across varying environmental conditions while preserving sensitivity to detect neurological changes, potentially eliminating the need for strict lighting standardization protocols.

As Ong and colleagues conclude, "Ambient light conditions can significantly alter quantitative pupillary measurements in both healthy and critically ill subjects. In order to produce maximally reliable and valid results, examiners should aim to standardize light levels when possible" (Ong et al.). Until lighting-corrected technologies become widely available, clinical teams must acknowledge that measurement variations directly impact patient care decisions in critical situations.

The evidence clearly indicates that ambient light effects represent more than a technical limitation—they constitute a patient safety concern that demands immediate attention through either rigorous protocol standardization or adoption of lighting-invariant measurement technologies.

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