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Mesopic vision
Mesopic vision
Mesopic vision
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Mesopic vision, sometimes also called twilight vision, is a combination of photopic and scotopic vision under low-light (but not necessarily dark) conditions.[1] Mesopic levels range approximately from 0.01 to 3.0 cd/m2 in luminance. Most nighttime outdoor and street lighting conditions are in the mesopic range.[2]

Human eyes respond to certain light levels differently. This is because under high light levels typical during daytime (photopic vision), the eye uses cones to process light. Under very low light levels, corresponding to moonless nights without artificial lighting (scotopic vision), the eye uses rods to process light. At many nighttime levels, a combination of both cones and rods supports vision. Photopic vision facilitates excellent color perception, whereas colors are barely perceptible under scotopic vision. Mesopic vision falls between these two extremes. In most nighttime environments, enough ambient light prevents true scotopic vision.

In the words of Duco Schreuder:

There is not one single luminescence value where photopic vision and scotopic vision meet. [Rather,] there is a wide zone of transition between them. Because it is between photopic and scotopic vision, it is usually called the zone of mesopic vision. The reason that the zone of mesopic vision exists is because the activities of neither cones nor rods is simply switched 'on' or 'off'. There are reasons to believe that the cones and the rods both operate in all luminescence conditions.[3]

As a result of gradually switching from cones to rods in processing light, a number of visual effects occur:[4]

  • The rods have a different wavelength sensitivity, causing blue objects to appear brighter and red objects to appear darker. This is called the "Purkinje shift".[4]: 7 
  • Color appears desaturated and hues change, drifting towards a dull purple.[4]: 8 
  • Spatial acuity decreases linearly with log-luminance. A varying "noise" slowly becomes more prominent.[4]: 8 

Cinematographers intentionally emulate mesopic effects to make scenes look darker than a display can actually achieve.[4]: 1 

Photometry

[edit]

The traditional method of measuring light assumes photopic vision and is often a poor predictor of how a person sees at night. Typically research in this area has focused on improving street and outdoor lighting as well as aviation lighting.

Prior to 1951, there was no standard for scotopic photometry (light measurement); all measurements were based on the photopic spectral sensitivity function V(λ) which was defined in 1924.[5] In 1951, the International Commission on Illumination (CIE) established the scotopic luminous efficiency function, V'(λ). However, there was still no system of mesopic photometry. This lack of a proper measurement system can lead to difficulties in relating light measurements under mesopic luminances[6] to visibility. Due to this deficiency, the CIE established a special technical committee (TC 1-58) for collecting the results of mesopic visual performance research.[7]

Two very similar measurement systems were created to bridge the scotopic and photopic luminous efficiency functions,[8][9][10] creating a unified system of photometry. This new measurement has been well-received because the reliance on V(λ) alone for characterizing night-time light illumination can result in the use of more electric energy than might otherwise be needed. The energy-savings potential of using a new way to measure mesopic lighting scenarios is significant; superior performance could in certain cases be achieved with as much as 30 to 50% reduction in the energy use comparing to the high pressure sodium lights.[11]

Mesopic luminosity function

[edit]

The mesoscopic luminosity function at wavelength can be written as the weighted sum,[12]

,

where is the standard photopic luminosity function (peaking at 683 lm/W at 555 nm) and is the scotopic luminosity function (peaking at approx. 1700 lm/W at 507 nm), and standardized by CIE and ISO.[13] The parameter is a function of the photopic luminance . Various weighting functions are in use, for blue-heavy and red-heavy light sources, as proposed by two organizations, MOVE and Lighting Research Center (LRC).[12]

MOVE and LRC weighting factors
Lp Blue-heavy Red-heavy
(cd/m2) MOVE LRC MOVE LRC
0.01 0.13 0.04 0.00 0.01
0.1 0.42 0.28 0.34 0.11
1.0 0.70 1.00 0.68 1.00
10 0.98 1.00 0.98 1.00

Note that the Lp–x curves defined by the two organizations have very different shapes. The weighted function is intended to define "visual effectiveness", i.e. how helpful a light source is in helping human spot an object, rather than any perceived level of luminance. Variation in how this "effectiveness" is rated causes variation of weights.[12]

References

[edit]

Further reading

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Mesopic vision

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from Grokipedia
Mesopic vision, also referred to as twilight vision, is the intermediate range of human that bridges photopic (daylight) and scotopic (nighttime) conditions, occurring at low light levels where both rod and photoreceptors in the are active and contribute to visual processing. This regime typically spans luminances from approximately 0.005 to 5 cd/m², encompassing common scenarios such as , dawn, and artificially lit nighttime environments like roadways. Physiologically, mesopic vision arises from the interaction between rod and signals in the retinogeniculate pathways, where provide increased light sensitivity through their higher amplification, while cones enable residual color discrimination and spatial acuity. Rod signals integrate into cone-dominated pathways via gap junctions and amacrine cells, leading to phenomena such as the Purkinje shift, where shorter-wavelength (bluish) colors appear brighter relative to longer-wavelength (reddish) ones as levels decrease. Spectral luminous efficiency in this range transitions from the photopic peak at 555 nm toward the scotopic peak at 507 nm, with the exact function depending on adaptation and the scotopic-to-photopic (S/P) ratio of the source. Mesopic vision significantly impacts visual performance, including reduced contrast sensitivity, altered temporal processing (e.g., slower reaction times by 8–20 ms compared to photopic conditions), and impaired chromatic discrimination, particularly along the tritan (blue-yellow) axis. These effects are critical for applications like outdoor , where standards such as CIE 191:2010 recommend mesopic photometry to optimize for tasks like , accounting for light source to enhance perceived without excessive energy use. Research also links mesopic deficits to conditions like age-related , highlighting its role in clinical assessments of function.

Definition and Characteristics

Luminance Range

Mesopic vision encompasses luminance levels approximately between 0.005 and 5 cd/m², as established by the (CIE) standards for transitional lighting conditions where both rod and photoreceptors contribute to . This range bridges , which predominates at higher intensities, and at lower ones, reflecting the intermediate adaptation state of the human visual system. The upper limit of this range, around 5 cd/m², aligns with the approximate threshold beyond which photopic processes fully take over and color discrimination becomes reliable. Conversely, the lower boundary near 0.005 cd/m² corresponds to the onset of significant involvement as rod responses approach their maximum sensitivity, transitioning away from purely rod-dominated . These boundaries are not absolute but represent practical delineations based on empirical measurements of visual thresholds. The precise extent of the mesopic range can vary due to factors such as adaptation time, which influences how quickly the shifts between regimes, and the spectral composition of the light source, which affects the relative stimulation of and cones through the scotopic-to-photopic (S/P) ratio. Longer adaptation periods may extend effective sensitivity into marginally lower luminances, while spectra with higher short-wavelength content can shift perceived boundaries by enhancing rod activation. This framework originated from early 20th-century psychophysical experiments, notably those by Selig Hecht in the 1920s and 1930s, who quantified how and sensitivity vary with illumination intensity, laying the groundwork for distinguishing mesopic conditions from adjacent regimes. Hecht's work, including measurements across log units of , demonstrated the gradual transition in visual function, influencing subsequent CIE definitions.

Visual Performance

In mesopic conditions, is markedly reduced compared to , with typical performance ranging from approximately 20/40 to 20/200 Snellen equivalents due to the mixed contributions from rod and photoreceptors. Studies on healthy subjects have shown that under mesopic levels around 0.1 cd/m², the average starting visual acuity deteriorates to about 0.39 logMAR (roughly 20/50 Snellen), while functional visual acuity, which accounts for dynamic viewing stability, reaches 0.52 logMAR (about 20/67 Snellen), and minimum acuity can drop to 0.78 logMAR (around 20/120 Snellen). This degradation is more pronounced and variable than in brighter photopic conditions (e.g., 250 cd/m²), where acuity often exceeds -0.10 logMAR (better than 20/16 Snellen), highlighting the instability of fixation and resolution in dimmer light. Color vision in mesopic environments is impaired, resulting in desaturated hues and a shift toward reliance on blue-yellow rather than the more precise red-green distinctions typical of . As decreases into the mesopic range, rod activation weakens cone-mediated chromatic signals, leading to reduced saturation of colors and poorer overall color , particularly along the red-green axis. losses are most evident near the tritan (blue-yellow) axis, where rod signals can either exacerbate or partially mitigate deficits depending on stimulus configuration, but red-green pathways suffer greater impairment overall. Mesopic vision benefits from rod contributions to peripheral sensitivity, supporting detection of moving stimuli in the visual periphery (e.g., at eccentricities of 10°) compared to central foveal processing, though motion integration can be impaired by temporal differences between rod and cone signals. This rod-mediated enhancement supports heightened peripheral awareness, crucial for tasks like navigating dim environments, though it comes at the cost of reduced central detail resolution. Contrast sensitivity functions under mesopic conditions exhibit a band-pass for achromatic stimuli, often peaking at intermediate spatial frequencies of 1–2 cycles per degree (cpd), which reflects the transitional dominance of rod inputs optimizing detection at these scales. For instance, at mesopic luminances such as 0.02–20 cd/, peak sensitivity occurs around these mid-range frequencies, differing from the low-pass profile seen in purely chromatic processing or higher photopic conditions where peaks shift to finer details. These metrics underscore how mesopic performance balances rod-enhanced sensitivity against limitations, with empirical thresholds showing improved detection for gratings at 1–6 cpd compared to scotopic extremes.

Physiological Mechanisms

Photoreceptor Involvement

In mesopic vision, which occurs at intermediate levels typically ranging from 0.001 to 3 cd/m², both rod and photoreceptors contribute to visual processing, with their relative involvement shifting based on light intensity. Rod photoreceptors, responsible for , exhibit exceptionally high sensitivity, capable of detecting single photons absorbed by their photopigment . These cells become active at luminances below approximately 0.01 cd/m², providing achromatic () input with relatively low spatial acuity due to greater convergence of rod signals onto bipolar cells. Cone photoreceptors, which mediate , have lower sensitivity but offer high spatial acuity and trichromatic color discrimination through their L-, M-, and S-opsins. Cones begin to activate significantly above about 0.1 cd/m², but in the mesopic range, they provide residual signals that supplement rod-dominated perception, particularly for finer details and subtle color cues under dimmer conditions. This dual activation enables a transitional visual experience where rod contributions predominate at the lower end of the mesopic spectrum, while cones gain influence as increases. A key phenomenon in this transition is the Purkinje shift, where the peak of the moves from yellow-green wavelengths (around 555 nm, cone-dominated ) to blue-green wavelengths (around 505 nm, rod-dominated ), most prominently around 1 cd/m². This shift arises from the differing absorption spectra of rod and cone opsins, enhancing sensitivity to shorter wavelengths as rods take precedence in lower mesopic light. The anatomical distribution of photoreceptors further explains rod dominance in low mesopic conditions. outnumber cones by a of approximately 20:1 across the human retina, with cones densely packed in the central fovea for high-acuity tasks and concentrated in the periphery, where they facilitate in dim . This peripheral rod enrichment underscores their role in providing broad, sensitive detection during mesopic viewing, such as in twilight environments.

Adaptation Processes

The adaptation of the to mesopic light levels involves a transition from cone-dominated to rod-influenced , characterized by the biexponential dark adaptation curve. This curve consists of an initial fast phase, primarily driven by cone photoreceptors, which recovers sensitivity within 5-10 minutes following light exposure, followed by a slower rod-mediated phase that can extend up to 30 minutes for full recovery. In the mesopic luminance range (approximately 0.001–3 cd/m²), these phases overlap, resulting in a hybrid sensitivity where both photoreceptor types contribute dynamically to visual thresholds. Central to this process is the biochemical recovery of photopigments after bleaching by light exposure. In , regenerates more slowly, with rates of approximately 1% per minute, requiring about 30-40 minutes for near-complete restoration after significant bleaching, which limits sensitivity gain in the early stages of . photopsins regenerate much more rapidly, achieving substantial recovery in 6-8 minutes, enabling quicker adjustment but with lower absolute sensitivity. This differential regeneration rate fosters the hybrid sensitivity observed in mesopic conditions, where incomplete rod recovery prolongs the reliance on residual function. At the neural level, adaptation in mesopic vision involves integrative responses in retinal cells, facilitated by mixed rod-cone pathways converging through bipolar cells. Rod signals primarily route via rod bipolar cells to AII amacrine cells, which then connect to cone bipolar cells, allowing indirect rod input to both ON and OFF ganglion cells; however, OFF pathways predominate in low mesopic due to their higher sensitivity to decrements. This convergence enables ganglion cells to process combined inputs, with rod contributions enhancing contrast detection as progresses. The speed of adaptation is modulated by background luminance, with higher mesopic levels (closer to photopic thresholds) slowing the overall recovery rate by maintaining partial bleaching and elevating thresholds. In brief exposures typical of dynamic mesopic environments, adaptation remains incomplete, limiting full rod involvement and preserving a cone-rod transitional state for several minutes.

Photometric Framework

Luminosity Functions

Luminosity functions in mesopic vision quantify the relative sensitivity of the human to different wavelengths of light under intermediate levels, where both cones and contribute significantly to . These functions bridge the photopic luminosity function V(λ), which characterizes cone-dominated daylight vision, and the scotopic function V'(λ), which describes -dominated night vision. In mesopic conditions, the is not fixed but varies with adaptation , necessitating adaptive models that account for the Purkinje shift—the transition in peak sensitivity from longer to shorter wavelengths as light levels decrease. The historical development of mesopic luminosity functions began with empirical data from H. R. Blackwell's 1946 study on contrast thresholds, which measured visual performance across luminances from 10^{-3} to 10^4 cd/m² using forced-choice methods on 7 observers, providing key insights into the transition zone despite focusing primarily on threshold detection rather than . Early models in the and built on this by proposing linear interpolations, but they overlooked non-linear rod-cone interactions, such as mutual inhibition and , leading to inaccuracies in predicting tasks like detection and reaction time. Modern CIE models evolved from these foundations, incorporating psychophysical data on visual performance to address non-linear effects, with the 2010 recommendation refining interpolation techniques based on extensive validation against Blackwell-derived and newer datasets. The CIE moveable luminosity function, V*(λ), provides a standardized approach by interpolating between V(λ) and V'(λ) according to the . It is mathematically expressed as Vmes(λ)=a(α)⋅V(λ)+[1−a(α)]⋅V′(λ)V_{\text{mes}}(\lambda) = a(\alpha) \cdot V(\lambda) + [1 - a(\alpha)] \cdot V'(\lambda) where α represents the , typically the base-10 logarithm of the photopic in cd/m² (ranging from approximately -2.6 to 2.1 for mesopic levels), and a(α) is a that decreases from 1 (fully photopic) to 0 (fully scotopic) as α declines, often computed via empirical fits like a(α) = 1 / (1 + exp(-k(α - α_0))) or tabular values to ensure smooth transition. This formulation allows the function to adapt dynamically, improving predictions for applications like road lighting where spectral content and vary. As adaptation shifts from photopic to scotopic, the peak wavelength of the luminosity function moves from 555 nm, where green-yellow light elicits maximum response in cones, to 507 nm, dominated by rod sensitivity in the blue-green spectrum; in mesopic conditions, this peak typically resides around 520 nm, reflecting a balanced rod-cone contribution that enhances sensitivity to shorter wavelengths compared to photopic vision. Despite their utility, these luminosity functions have limitations, as they presume steady-state adaptation where the visual system has equilibrated to constant luminance, rendering them less accurate for transient scenarios involving sudden luminance changes, such as flashing lights or rapid eye movements, where temporal adaptation dynamics introduce additional variability in perceived brightness.

Measurement Standards

The (CIE) established a foundational standard for mesopic photometry in CIE 191:2010, titled "Recommended System for Mesopic Photometry Based on Visual Performance," which provides a unified framework for quantifying visual stimuli across light levels by integrating photopic and scotopic contributions. This standard defines equivalent mesopic (L_mes) as a function of photopic (L_p) and the scotopic-to-photopic (S/P) ratio of the light source, enabling the calculation of efficacy factors particularly relevant for energy-efficient LED applications in road lighting, where mesopic conditions prevail. It emphasizes visual performance metrics, such as threshold contrast sensitivity, to correlate measurements with human under luminances from 0.005 to 5 cd/m². To implement these standards, instrumentation relies on spectroradiometers, which capture the (SPD) of light sources to compute mesopic (E_mes) or by convolving the SPD with the mesopic V(λ)_mes. These devices, calibrated across visible wavelengths (typically 380–780 nm), allow precise derivation of S/P ratios and states, essential for outdoor environments like roadways where spectral content varies with technology. Conversion factors in mesopic photometry bridge photopic measurements to perceived using approaches rooted in visual thresholds, such as the Blackwell threshold method, which models contrast detection at mesopic levels to adjust for rod-cone interplay. CIE 191:2010 outlines two primary methods: an iterative procedure solving for based on the visual field's average S/P ratio, and a direct approximation for simpler applications, both yielding equivalent values that can reduce overdesign in photopic-based systems by up to 30–40% for white LEDs. These factors account for the shift in peak sensitivity toward shorter wavelengths in mesopic vision, improving correlations between measured light levels and observer performance. Post-2020 developments have refined these protocols, with ISO/CIE 23539:2023, "Photometry — The CIE of physical photometry," standardizing the CIE photometry , including the mesopic luminous efficiency functions V_mes;m(λ) with adaptation parameter m derived from and S/P ratios. As of 2025, this continues to be applied in street lighting evaluations.

Practical Applications

Environmental Contexts

Mesopic vision predominates in various natural environments where ambient light levels fall within the intermediate range of approximately 0.01 to 3 cd/, enabling a blend of rod and activity. During civil twilight, defined as the period when the sun is between 0° and 6° below the horizon, typically spans 0.01 to 1 cd/, marking the transition from photopic vision to darker conditions. This phase occurs universally and , with similar profiles in forested understories, where dense canopies reduce direct to mesopic levels even during daylight hours, fostering to dim, diffused illumination. Moonlit nights further exemplify these conditions, particularly under a , when clear reaches 0.02 to 0.05 cd/, sufficient for basic visibility but reliant on both photoreceptor types. Artificial environments also frequently invoke mesopic vision, especially in controlled low-light settings designed for ambiance or preservation. Indoor spaces like museums and theaters maintain wall and display luminances below 3 cd/ to protect sensitive artifacts or enhance dramatic effects, placing observers in a transitional visual state. Urban streets with partial or residential lighting represent another common scenario, where road surface luminances range from 0.3 to 2 cd/ under typical nighttime conditions, blending artificial sources with ambient darkness. Temporally, mesopic vision aligns with daily cycles during dawn and dusk transitions, which last 30 to 60 minutes depending on latitude and season, allowing gradual ocular adaptation. In higher latitudes, such as above 50°, these periods extend seasonally—up to several hours during midsummer solstice due to the sun's shallow horizon angle—prolonging exposure to mesopic conditions and influencing circadian rhythms. In daily life, mesopic vision's prevalence underscores its role in safety-critical activities like , where dawn and dusk periods account for about 5% of all fatalities despite their brevity, indicating disproportionately higher crash risks compared to daylight hours. Nighttime accidents overall, often under mesopic lighting from streetlamps, comprise roughly 50% of fatal crashes while representing only 25% of travel volume, highlighting the challenges of visual performance in these settings.

Design and Safety Implications

In roadway lighting design, the Commission Internationale de l'Éclairage (CIE) guidelines, particularly CIE 191:2010 on mesopic photometry, recommend accounting for the enhanced sensitivity of the in low-light conditions to optimize pole efficacy and reduce . By leveraging mesopic vision principles, lighting installations can achieve higher , allowing for fewer or lower-wattage fixtures while maintaining , with reported energy savings of 20-30% through the use of blue-enriched spectra that shift the scotopic/photopic (S/P) ratio favorably. Aviation and maritime safety protocols incorporate mesopic vision considerations to enhance contrast in low-luminance environments. The (FAA) standards in 150/5340-30J specify visual aids, such as edge lights and centerline lights, that support operations in low- conditions (e.g., below 1200 ft RVR), ensuring pilots can detect edges and obstacles through optimized contrast ratios during nighttime or fog-reduced . Similarly, the () guidelines for bridge lighting, as outlined in MSC/Circ.982 on ergonomic criteria, emphasize maintaining low levels (0.05–0.15 , mesopic range) to preserve dark adaptation for lookouts, thereby improving the detection of navigational lights and hazards via heightened peripheral contrast sensitivity. For visual comfort in mesopic office environments, post-2015 LED innovations have focused on glare mitigation through diffused and tunable spectra, aligning with the transition to that reduces discomfort while supporting hybrid rod-cone function. These advancements, such as anti-glare coatings and lower blue-light peaks in tunable white LEDs, enable dynamic adjustment to mesopic levels around 1-5 cd/, minimizing visual in dim workspaces without compromising task performance. Emerging trends in adaptive lighting systems utilize sensors for real-time mesopic adjustments, particularly in pilots funded by EU Horizon programs. Projects like SCALS (Smart Cities Adaptive Lighting System) under Horizon 2020 demonstrate sensor-driven modulation of and spectra based on ambient conditions, achieving up to 50% energy efficiency gains while enhancing safety in transitional lighting zones. Similarly, the ENLIGHTENme initiative, concluded in 2025, integrated health-focused metrics into adaptive controls, piloting IoT-enabled streetlights that respond to and twilight to optimize mesopic .

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