Stilb (sb): High-Luminance Legacy Unit for Display and Optical Benchmarks
The stilb (symbol sb) measures luminance in the CGS system. One stilb equals one candela per square centimetre, or 10 000 cd·m⁻². Because the stilb references a square-centimetre area, it naturally appears in optical bench setups, astronomical brightness tables, and mid-twentieth-century display specifications. Translating stilb values into SI nits (cd·m⁻²) ensures that archival data and modern measurements can be compared on equal footing.
Cross-reference this explainer with the candela-per-square-metre article and experiment with lighting layouts using the room lighting calculator to keep luminance, illuminance, and flux discussions synchronised.
Definition and Unit Relationships
CGS formulation of luminance
Luminance quantifies luminous intensity per projected area in a specified direction. In CGS photometry, luminous intensity is measured in candles (the CGS analogue of the candela), and area is measured in square centimetres. One stilb therefore equals one candle per square centimetre. Converting to SI, note that 1 cm² equals 10⁻⁴ m² and that the modern candela supersedes the candle. The resulting relation is 1 sb = 10 000 cd·m⁻². Conversely, 1 cd·m⁻² equals 10⁻⁴ sb. Because many modern displays operate between 200 and 1 000 cd·m⁻², their luminance ranges correspond to 0.02 to 0.10 stilb.
Position within the CGS photometric family
The stilb complements the lambert and apostilb. Whereas the stilb uses the projected area of the luminous surface, the lambert incorporates the cosine law for perfectly diffuse emitters. Specifically, 1 lambert equals 1/π stilb, while 1 apostilb equals 10⁻⁴ stilb. Recognising these relationships streamlines conversions among legacy documents. Conservation labs, for example, often encounter lamberts in exhibit logs and stilbs in instrument manuals. Maintaining a conversion table that lists stilb, lambert, apostilb, and cd·m⁻² helps avoid confusion when preparing reports for international audiences.
Historical Context
Rise of CGS photometry
Nineteenth-century photometry sought coherence across national laboratories. The CGS system provided a convenient scaffold, defining derived photometric units in terms of centimetres, grams, and seconds. The stilb emerged as the luminance unit during the 1920s when astrophysicists and lighting engineers required a high-intensity metric. Publications by the International Commission on Illumination (CIE) catalogued luminance standards in stilbs, supporting comparisons of cathode-ray tube screens, projection systems, and celestial objects.
Shift toward the candela per square metre
The General Conference on Weights and Measures formalised the candela in 1948 and promoted the square metre as the reference area. By the 1970s, the term “nit” (1 cd·m⁻²) dominated lighting and display industries. Nonetheless, stilbs persisted in astronomy, museum conservation, and military display specifications because existing datasets, calibration equipment, and legal documents were written in CGS terms. Understanding the stilb is therefore vital when digitising archives or reviewing long-running experiments.
Concepts and Measurement Techniques
Radiometry versus photometry
Luminance is a photometric quantity weighted by the CIE photopic luminosity function V(λ). When converting radiometric data (W·sr⁻¹·m⁻²) to stilbs, apply spectral weighting and convert watts to lumens using the 683 lm·W⁻¹ factor at 555 nm. Instruments such as spot photometers or imaging colorimeters may output results in cd·m⁻² by default; to obtain stilbs, divide by 10 000. Accurate measurements require cosine-corrected detectors and precise knowledge of the viewing geometry, topics expanded in the lux article.
Calibration strategies
Calibration laboratories maintain luminance standards using integrating sphere sources, standard lamps, or LED-based transfer standards traceable to national metrology institutes. When clients request certificates in stilbs, technicians convert SI results, propagate uncertainties, and document correlation terms. Metrologists ensure that colour temperature, spectral mismatch, and detector linearity are accounted for, especially when comparing CRT-era references (often stated in stilb) with modern solid-state devices.
Applications Across Disciplines
Display engineering and evaluation
Early cathode-ray tube and oscilloscope displays regularly cited luminance in stilbs because the centimetre-scale phosphor screen area matched the CGS definition. Even today, engineers refurbishing heritage avionics or oscilloscopes encounter stilb-based limits for legibility and blooming. Translating those thresholds to cd·m⁻² supports compatibility with contemporary standards such as MIL-STD-3009 for night-vision imaging system (NVIS) compatibility.
Astronomy and sky brightness measurements
Astronomers historically reported surface brightness of nebulae and galaxies in magnitudes per square arcsecond or in stilbs. Converting stilb values to SI luminance simplifies comparisons with space-based instruments calibrated in W·m⁻²·sr⁻¹. Amateur observatories digitising photographic plates often record both units, ensuring that modern photometric pipelines can ingest archival observations without ambiguity.
Museum conservation and heritage lighting
Museum guidelines limit exhibit luminance to protect sensitive artifacts. Conservation records from the mid-twentieth century frequently specify stilb values for showcase lighting. When updating conservation plans, practitioners convert those numbers to cd·m⁻² or lux using reflectance models. Tools like the lux-to-lumens calculator help translate historic lighting budgets into contemporary fixture selections.
Importance for Modern Practice
Data preservation and interoperability
Digitisation projects must faithfully reproduce original measurement units. By understanding stilb relationships, archivists can annotate metadata with both CGS and SI equivalents, enabling long-term interoperability. Standards bodies encourage dual reporting when legacy datasets underpin regulatory or scientific decisions, mirroring the approach recommended in the ISO 80000-7 overview.
Communication with multidisciplinary teams
Lighting designers, astronomers, conservationists, and display engineers may prefer different units. Presenting luminance data simultaneously in stilb and cd·m⁻² ensures that each stakeholder can reference familiar numbers without misinterpretation. Combining these figures with qualitative descriptors—such as glare probability or colour rendering—provides a complete picture of visual performance.
Mastering the stilb therefore strengthens cross-generational collaboration, safeguards archival insights, and enriches modern photometric analysis. When paired with SI fundamentals, the legacy unit remains a valuable interpretive tool.