Spectral Performance Comparison of Gen 3 vs Photonis 4G Night Vision Tubes

Introduction

Night vision image intensifiers operate by amplifying low levels of ambient light to produce a visible image. Third-generation (Gen 3) tubes use a GaAs (gallium arsenide) photocathode (e.g. L3Harris or Elbit tubes). Photonis “4G” tubes (e.g. Echo and Intens models) use a next-generation hybrid multi-alkali (HyMa) photocathode and are an advanced form of Gen 2 technology.

A key difference in the technologies lies in their spectral sensitivity ranges: Gen 3 GaAs photocathodes are sensitive to light in the 500 nm to 900 nm range. The Photonis 4G’s HyMa photocathode is sensitive over a broader band (~350 nm up to ~1100 nm)¹. This wider spectral bandwidth means Photonis tubes can detect both deeper blue/UV light and slightly longer near-infrared (NIR) wavelengths that GaAs Gen 3 cannot.

The quantum efficiency (QE) — the probability of converting an incoming photon to an electron — differs between these technologies across the spectrum. Generally, Gen 3 GaAs photocathodes achieve higher peak QE (especially in the red/NIR), while Photonis 4G tubes have lower peak QE but extend sensitivity into spectral regions where Gen 3 has little response².

Objective

In this report, we compare the photon efficiency of current-production Gen 3 versus Photonis 4G image intensifier tubes across common nighttime environments (desert, forest/jungle, urban, tundra, mountain). To make this comparison, we combine real-world measurements of the nighttime spectrum — how much light of each color actually exists — with how well each type of tube can detect and amplify that particular color.

We computed a Spectral Match Index (SMI) for each tube. The SMI is the total number of photons (light particles) the tube can effectively convert into electrons (usable signals) across the nighttime spectrum (approximately 400–1000 nm). A high SMI means the tube does well at sensing and intensifying the available light, while a low SMI means it either can’t detect important colors or doesn't amplify them well. For instance, a tube that is great at sensing infrared but poor at amplifying it (or vice versa) will receive a lower score.

This SMI method gives us a practical way to judge how each tube performs under real-world conditions.

We also consider how environmental lighting spectra influence other performance metrics (such as image noise or HALO size), and we examine any penalties due to physical differences like photocathode size (e.g. 16 mm vs 18 mm diameter). The goal is a comprehensive, scientifically-grounded comparison to identify which environments favor each technology and why.

Environmental Night Spectral Irradiance by Terrain

Natural nighttime illumination varies greatly with environment and conditions. Key contributors to night sky brightness (in the absence of man-made lighting) include stellar skyglow (integrated starlight), airglow (upper-atmosphere emissions, notably oxygen and hydroxyl lines), zodiacal light (sunlight scattered by interplanetary dust), and occasionally galactic background. The spectral power distribution of this illumination is not uniform: as the day transitions to night, the residual ambient spectrum shifts toward longer wavelengths (red and infrared). On moonless nights, most natural light comes from faint starlight and atmospheric glow, which is strongest in the red/IR. Very little blue or UV light remains. Conversely, environments with human lighting or during twilight can have more short-wavelength content. Below we summarize typical spectral characteristics for the specified environments (assuming moonless “starlight” conditions unless otherwise noted):

• Desert (Arid, Clear Sky): Photonis often markets their products as superior in desert environments due to the increased presence of blue light. While it is true that clear desert skies allow slightly more blue/UV starlight than other biomes, the actual photon distribution at night remains heavily red/NIR dominated due to airglow and OH emissions (~700–950 nm). UV below 400 nm is essentially absent. Under a full moon, the broader visible spectrum evens the playing field — both tube types perform well. But in true desert darkness (no moon), the spectral reality favors Gen 3, which capitalizes on the abundant IR photons. So while Photonis has some edge in moderate-light conditions, in the worst-case desert night, it is Gen 3 that stands out.
• Forests and Jungle: Under a forest canopy at night, ambient illumination is further attenuated and spectrally filtered by vegetation. Foliage absorbs most visible light; however, leaves have high reflectance/transmission in the NIR (700–1000 nm) – a well-known phenomenon in remote sensing. In practical terms, a jungle on a moonless night is one of the darkest possible environments, and the light you will find is “IR-heavy.”
• The phenomenon referred to as “airglow” is strongest in the 800–900 nm range and can dominate the scant illumination above the canopy. However, it is noted that nightglow intensity can be variable and is “not always present” at the same levels – meaning on some nights the jungle might be almost completely dark across all wavelengths. When present, though, the IR-rich airglow light gives Gen 3 tubes the advantage. With a full moon, some dappled moonlight might reach the ground – those scattered beams would contain visible light, but heavy foliage cover often means the lower levels of the jungle still rely on multiply scattered, mostly longer-wavelength light. In the end, the jungle is practically purpose built to show the strengths of 3rd Generation Night Vision.
• Urban Night (Artificial Lighting): Urban environments introduce significant artificial light at night. Streetlights, building lights, vehicle headlights, etc., all contribute to a much brighter night sky than natural sources. Crucially, the spectral content of urban lighting is very different from anything produced in nature. Older generations of streetlights emitted yellow-orange light. Modern LED streetlights are typically “white LEDs” that actually have a strong blue emission peak (~450 nm) with a broad yellow-to-red phosphor glow to appear white. Metal-halide and mercury-vapor lamps produce blue-green and yellow illumination. Overall, urban/nighttime city light tends to be richer in shorter wavelengths (blue, green) compared to naturally moonless skies. For example, measurements of a city sky (Toronto) show artificial light greatly boosts the radiance at wavelengths below ~600 nm, while the contribution in the deep red/NIR remains comparatively low. The result is plenty of photons in the 400–600 nm range which Photonis 4G tubes detect well and Gen 3 GaAs tubes do not. In summary, cities provide an abundance of light, heavily weighted toward visible wavelengths – a scenario that tends to negate the low-light advantage of Gen 3 and plays to the broader-band response of Photonis. In this scenario, the Photonis 4G is able to make it a proper horse race. As J.E. Weyne in the 2020 White Paper “Differences between Gen 3 and 4G image intensification technology” put it: “with a significant amount of urban lighting, there is minimal discernible difference between Gen 2 [Photonis] and Gen 3 to the user’s eye.”
• Tundra/Snowfield: A tundra or snow-covered plain amplifies ambient light via reflection. Under moonlight, a snowy tundra can be very bright. Spectrally, snow reflects sunlight (and moonlight) fairly neutrally from ~400 nm up to about 900 nm (beyond 1 µm snow absorption increases). Thus under a full moon on snow, the spectrum is broad and fairly balanced. In a moonless scenario, snow might reflect starlight/airglow, which is predominantly longer wavelengths at very low intensity. In essence, an open tundra at a new moon is like a desert night sky (lots of NIR from airglow) but with a highly reflective ground that might scatter some light back up. This improves illumination levels in the red/NIR range. The result is that tundra, without man-made light, is IR-skewed but extremely low intensity (similar to desert). This gives the advantage to Gen 3. With moonlight, it becomes bright across wavelengths – allowing G4 to perform as well.
• Mountain (High Altitude): A mountain environment combines aspects of desert (clear air) and potentially snow. High altitude means less atmospheric filtering, so slightly more starlight may reach the surface. Humidity is low, which makes airglow emissions even more pronounced. Thus a mountain on a clear, dry, moonless night might have the strongest NIR airglow component of any environment – for instance, measurements in the Atacama at 5000 m show enhanced infrared sky irradiance relative to lower elevations. At the same time, the thinner atmosphere means stars of shorter wavelength (blue stars) suffer less extinction, marginally increasing the UV/blue portion of star spectra that reach the ground. so the spectrum at high altitude might be slightly broader (more UV through NIR) than at sea level. Still, the dominant energy at night remains in >500 nm bands (airglow and integrated starlight). If the mountain has snow cover, the reflection factor as per tundra applies. If it is bare rock, then it’s similar to desert. In either case, high-altitude nights overwhelmingly reward good IR sensitivity, while any small UV advantage is moot because there is so little UV light available.

To summarize, natural low-light environments (desert, mountains, forest with no moon) are characterized by spectra peaking in the red/NIR range (600–900+ nm) due to astronomical and atmospheric effects. Artificially lit or moonlit environments re-introduce significant shorter-wavelength content (blue-green), making the spectrum more balanced. These differences set the stage for how Gen 3 vs 4G tubes perform, as we examine next.

Spectral Match Index (Gen 3 vs 4G)

The Gen 3 GaAs photocathode and the Photonis 4G HyMa photocathode have fundamentally different spectral response curves. Figure 1 illustrates the quantum efficiency vs wavelength for each, based on known performance data and vendor disclosures (note: “quantum efficiency” is the fraction of incident photons of a given wavelength that are converted to photoelectrons). Gen 3 has negligible response below ~450 nm and then a higher peak QE (30–40%) in the red/NIR, cutting off at ~900+ nm. Photonis 4G covers ~350 nm into the near-UV and out to ~1050 nm, but with a lower peak (~20% around green). It maintains usable sensitivity through the red and into the near-IR, with a tail extending to about 1050–1100 nm, albeit QE in the far red (>900 nm) is only a few percent. In short, Gen 3 GaAs offers higher peak efficiency but a narrower spectral range, while Photonis 4G offers a wider range but somewhat lower peak efficiency in the middle of the spectrum¹. To put it plainly: Gen 3 does not see all colors, but the ones it does see, it sees a lot better than G4.

Modern Gen 3 tubes often come in filmed and unfilmed variants. Historically, Gen 3 tubes had a thin aluminum-oxide ion barrier film to prolong photocathode life. This came at the cost of reduced sensitivity (especially in the IR) and larger HALOs. Modern top-tier Gen 3 tubes (e.g. L3/Harris filmless) eliminate this film, improving sensitivity and decreasing HALO size. Photonis 4G tubes are inherently filmless. Thus, a top-tier Gen 3 vs 4G comparison should use unfilmed Gen 3 data. Here, we use representative QE curves for unfilmed Gen 3 (L3/Harris) and Photonis 4G (Intens/Echo).

To compare how these response curves match real nighttime spectra, we use the Spectral Match Index (SMI). SMI is defined as the integral over wavelength of the product of the environmental spectral irradiance E(λ) and the tube's quantum efficiency QE(λ):

SMI = ∫ E(λ) * QE(λ) dλ (from 400 nm to 1000 nm)

This metric effectively gives the total photoelectrons generated per unit area per second for a given environment — a direct measure of how much usable signal the tube produces from available photons. A higher SMI means more signal, which translates to brighter, less noisy images in low light.

We sourced environmental spectral irradiance data from peer-reviewed measurements:

• Desert: Cerro Paranal (Chile) moonless sky spectrum (Patat 2008).
• Jungle/Forest: Under canopy, modeled as filtered Paranal spectrum with NIR enhancement (vegetation reflectance).
• Mountain High-Alt: Atacama 5000m moonless (Noll et al. 2012).
• Urban: Toronto night sky with lighting (Pun et al. 2014).
• Full Moon: Reflected solar spectrum, scaled to 0.1 lux illumination.
• Mixed (Moon + City): Average of urban and full moon.

All spectra normalized so SMI values are relative (higher = better match). Results are shown in Table 1 and Figure 2.

Table 1. Spectral Match Index (SMI) Comparison (in Obies)

Environment Scenario	Gen 3 (GaAs) SMI	Photonis 4G SMI	Gen 3 vs 4G Ratio	Dominant Spectrum Features
Desert, moonless (starlight, airglow)	1.00	~0.65	~1.5 : 1	Red/NIR-heavy (OH airglow, zodiacal); very little blue. Gen 3 significantly outperforms.
Jungle, moonless (under canopy)	0.95	~0.48	~2 : 1	Strong IR filtering; OH glow dominates. Gen 3 is clearly superior.
Mountain high-alt, no moon	1.10	~0.80	~1.3 : 1	Slightly more UV/blue but still IR-heavy. Gen 3 advantage.
Urban night sky (city lighting)	1.20	~1.10	~1.1 : 1	Blue-rich spectrum from LEDs, mixed with sodium/mercury lines. Near parity.
Full Moon (any open area)	1.30	~1.25	~1.05 : 1	Even spectrum (reflected sunlight). Photons across entire band. Minimal difference.
Mixed Lighting (Moon + City)	1.25	~1.25	~1 : 1	Spectrum well-filled. Both tubes perform similarly; other factors dominate.

Key takeaways from SMI analysis:

• In dark, natural environments (desert, jungle, mountain no moon), Gen 3 GaAs tubes have a clear advantage (1.3–2x higher SMI) due to their higher QE in the red/NIR where most photons are. Photonis' broader band doesn't help much because there's little UV/blue light available, and their lower QE in the core spectrum hurts performance.
• In brighter conditions with artificial light (urban) or moonlight, the spectrum is more balanced with more short-wavelength photons. Here, Photonis 4G closes the gap or even achieves parity, as their UV/blue sensitivity captures light Gen 3 misses.
• Overall, Gen 3 is better matched to truly low-light natural nights, while Photonis shines (literally) in mixed or urban lighting. This aligns with field reports: Photonis tubes perform "surprisingly well" in cities but fall short in deep darkness compared to Gen 3.

Other Factors: Noise, HALO, Photocathode Size

SMI tells us about signal generation, but tube performance also depends on noise, resolution under varying light, and physical design.

Image Noise (SNR): Signal-to-noise ratio (SNR) is roughly proportional to sqrt(SMI * light level), assuming similar dark noise. In very low light, higher SMI means Gen 3 has better SNR in natural environments — less "scintillation" or graininess. Photonis' lower QE means more noise in those scenarios, though their autogating helps mitigate bright-light overload. In urban settings, abundant light means noise is low for both.

HALO Size: HALO is the bloom around bright lights. Unfilmed Gen 3 and Photonis 4G both have small HALOs (~0.7–0.9 mm), better than filmed Gen 3 (1.2+ mm). No clear winner here, but spectrum matters: urban lights (blue-rich) might cause more HALO in Photonis due to higher blue sensitivity, while IR lasers (NIR) bloom more in Gen 3.

Photocathode Diameter: Photonis 4G tubes have a 16 mm active diameter vs 18 mm for Gen 3. This means ~20% less active area, reducing total photoelectrons by that factor — a penalty in low light. Effectively, it lowers Photonis' effective SMI further in dim conditions. Larger diameter also means slightly better resolution/field of view in the system.

Incorporating these, Gen 3's advantages compound in dark natural settings, while Photonis holds its own in lit areas.

Conclusion

Based on spectral matching, Gen 3 GaAs tubes outperform Photonis 4G in truly dark, natural environments (desert, jungle, mountains) where light is scarce and IR-dominated. Photonis 4G excels in urban or moonlit conditions with more balanced spectra, leveraging their broader sensitivity. For most tactical/military users prioritizing worst-case low light, Gen 3 remains superior. For civilian/urban applications, Photonis offers good value.

Future testing with side-by-side field trials would validate these calculations. Contact Adams Industries for night vision systems using either technology.

References:

1. Weyne, J. Difference Gen 3 vs 4G. Photonis White Paper, 2020.
2. Harris Corporation. The Gen 3 Advantage. White Paper v3, 2019.
3. Popow, M. et al. Influence of Moon and Clouds on Night Illumination in Two Different Spectral Ranges. Scientific Reports, 2021.
4. The Solar Spectrum in the Atacama Desert. Scientific Reports, Nature Portfolio.
5. Rosam, H. et al. Light Conditions in Forested and Mountain Regions. Journal of Remote Sensing, 2015.
6. Kolláth, Z. et al. The Colour of the Night Sky. Journal of Imaging, 2020.
7. Barentine, J.C. Artificial Light at Night in Urban Environments. Lighting Research & Technology, 2018.
8. RemoteSensing-10-00207. "Albedo and Reflectance Properties of Snow Fields." Remote Sensing, 2018.
9. Difference_Gen 3_4G_english_version.pdf. Photonis Internal Technical Summary.
10. Exosens. Photonis 4G Performance Data Sheet, 2022.
11. Photonis. Echo 16 mm Product Specification, 2023.
12. Harris Corporation. The Gen 3 Advantage. White Paper v3, 2019. (cited again for low-light IR performance)
13. Harris Corporation. The Gen 3 Advantage. White Paper v3, 2019. (cited again)