stilbs (sb) to foot-lamberts (fL) conversion

1 sb = 2918.635 fLfL → sb
Formula
1 sb = 2918.635 fL

Understanding Stilbs to Foot-Lamberts Conversion

The stilb (sb) is a CGS luminance unit equal to one candela per square centimeter. The foot-lambert (fL) is an imperial luminance unit equal to (1/π) candela per square foot, still used in cinema projection and display calibration to specify screen brightness. Converting stilbs to foot-lamberts is common when bright emitter data must be expressed in the units projectionists and AV engineers work with.

Conversion Formula

1 sb=2918.64 fL1\ \text{sb} = 2918.64\ \text{fL}

To convert stilbs to foot-lamberts, multiply by this factor:

fL=sb×2918.635\text{fL} = \text{sb} \times 2918.635

Step-by-Step Example

Convert 25 stilbs to foot-lamberts.

fL=25×2918.635=72965.9 fL\text{fL} = 25 \times 2918.635 = 72965.9\ \text{fL}

How to Convert Stilbs to Foot-Lamberts

Express a bright-surface luminance in the imperial unit used for screen calibration.

  1. Take the stilb value: For example, 25 sb.
  2. Multiply by 2918.635: This factor combines the area change and the 1/π foot-lambert definition.
  3. Compute: 25×2918.635=72965.925 \times 2918.635 = 72965.9.
  4. State the result: 25 stilbs equals about 72,965.9 fL.

stilbs to foot-lamberts conversion table

stilbs (sb)foot-lamberts (fL)
00
12918.635
25837.27
38755.905
411674.54
514593.18
617511.81
720430.45
823349.08
926267.72
1029186.35
1543779.53
2058372.7
2572965.88
3087559.05
40116745.4
50145931.8
60175118.1
70204304.5
80233490.8
90262677.2
100291863.5
150437795.3
200583727
250729658.8
300875590.5
4001167454
5001459318
6001751181
7002043045
8002334908
9002626772
10002918635
20005837270
30008755905
400011674540
500014593180
1000029186350
2500072965880
50000145931800
100000291863500
250000729658800
5000001459318000
10000002918635000

What is the stilb?

The stilb is a CGS unit of luminance, describing the brightness of a surface in terms of luminous intensity per unit area. It was widely used in older photometric literature before the SI candela per square metre became standard.

Definition

One stilb is one candela per square centimetre. Since there are ten thousand square centimetres in a square metre, the stilb is a large unit relative to the SI luminance unit.

1 sb=10000 cd/m21\ \text{sb} = 10000\ \text{cd/m}^2

Exactly, 1 sb=1 cd/cm2=104 cd/m2=104 nits1\ \text{sb} = 1\ \text{cd/cm}^2 = 10⁴\ \text{cd/m}^2 = 10⁴\ \text{nits}.

Origin and History

The stilb was introduced by the French physicist André Blondel around 1920, its name taken from the Greek stilbein, "to glitter." It belonged to the centimetre–gram–second system and was used chiefly in continental Europe.

Law and Notable Facts

The stilb is not part of the SI and has largely fallen out of use in favour of the candela per square metre. Because it references the square centimetre, a single stilb represents a very bright surface: the sun's disc at the horizon is on the order of a few hundred stilbs.

Real-World Examples and Conversions

  • A frosted incandescent lamp surface: a few stilbs.
  • The sun's disc viewed from Earth: roughly 160,000 cd/m², or about 16 stilbs.
  • 1 sb=104 cd/m2=104 nits1\ \text{sb} = 10⁴\ \text{cd/m}^2 = 10⁴\ \text{nits}.
  • 1 cd/m2=10−4 sb1\ \text{cd/m}^2 = 10⁻⁴\ \text{sb}.

What is the Foot-Lambert?

The foot-lambert is a non-SI unit of luminance, expressing the brightness of a surface as seen by an observer. It remains widely used in the cinema, projection, and display industries in the United States, where screen brightness is often specified in foot-lamberts.

Definition

The foot-lambert is defined so that a perfectly diffusing (Lambertian) surface emitting or reflecting a total luminous flux of one lumen per square foot has a luminance of one foot-lambert.

1 fL=3.42626 cd/m21\ \text{fL} = 3.42626\ \text{cd/m}^2

Equivalently, 1 fL=1π cd/ft21\ \text{fL} = \frac{1}{\pi}\ \text{cd/ft}^2. As with the lambert, the factor of π\pi arises from the cosine emission geometry of a Lambertian source. Because one square foot equals 0.09290304 m20.09290304\ \text{m}^2, the conversion follows directly: 1π×0.09290304 cd/m2=3.42626 cd/m2\frac{1}{\pi \times 0.09290304}\ \text{cd/m}^2 = 3.42626\ \text{cd/m}^2.

Origin and History

The foot-lambert is the imperial-unit counterpart of the lambert, both descending from Johann Heinrich Lambert's foundational photometry. It became entrenched in mid-20th-century American engineering practice, particularly in cinema, where SMPTE standards long specified projected picture brightness in foot-lamberts.

Law and Notable Facts

The foot-lambert is not an SI unit, but it persists in professional standards. SMPTE recommends an open-gate (unmodulated) screen luminance of 14 fL (≈ 48 cd/m²) for film projection and 16 fL peak white for digital cinema in a dark theater. HDR home displays, by contrast, target hundreds to over a thousand cd/m².

Real-World Examples and Conversions

  • 14 foot-lamberts (SMPTE film reference) ≈ 47.97 cd/m².
  • 1 foot-lambert ≈ 3.42626 cd/m² ≈ 0.001076 lambert.
  • A typical office display of 250 cd/m² is about 73 foot-lamberts.
  • 1 lambert ≈ 929.03 foot-lamberts.

Frequently Asked Questions

How many foot-lamberts equal one stilb?

One stilb equals 2,918.635 foot-lamberts.

How do I convert foot-lamberts back to stilbs?

Multiply the foot-lambert value by 0.0003426259. So 2,918.635 fL equals 1 sb.

Where are foot-lamberts used?

Foot-lamberts are standard in cinema and home-theater calibration; SMPTE recommends about 14 fL for film projection on screen.

Why does the foot-lambert include a 1/π factor?

The foot-lambert is defined so that a perfectly diffuse surface emitting one lumen per square foot has a luminance of one fL, which introduces the 1/π factor relative to candela-per-area units.

What is 2 stilbs in foot-lamberts?

Multiply 2 by 2,918.635 to get 5,837.27 fL.

People also convert

sb to cd/m2sb to ntsb to asbsb to L

Complete stilbs conversion table

sb
UnitResult
candelas per square metre (cd/m2)10000 cd/m2
nits (nt)10000 nt
apostilbs (asb)31415.93 asb
lamberts (L)3.141593 L
foot-lamberts (fL)2918.635 fL

Luminance conversions