lux = nit * π 尼特(nit)与勒克斯(lux, lx)的转换

lux = nit * π

测光表是摄影摄像环境用的，分入射、反射测光表。亮度计用来测量发光物体的光亮度。 照度计测量环境光。便宜的照度计只要40多元。

How to measure luminance of a monitor screen (NITS) with a lux meter (LUX)? - Physics Stack Exchange

Lambert's cosine law

也许可以这样理解：平板显示器的nit描述每个点的发光强度。我们看显示器或用照度计测量时，不同的点都有不同的贡献（直射和斜射之类）。

A foot-candle (or foot-candle, fc, lm/ft², or ft-c) is a measurement of light intensity. One foot-candle is defined as enough light to saturate a one-foot square with one lumen of light. The illuminance in lux is calculated by dividing the illuminance in Footcandles by 0.09290304, 或者说1 lux = 10.76 fc. 1米=3.2808399英尺，3.28083992≈10.76， 所以：

1 candle = 1 lumen

Candela (symbol: cd) is the base unit of luminous intensity in the International System of Units (SI). The lumen (symbol: lm) [流明] is the SI derived unit of luminous flux [通量], a measure of the total quantity of visible light emitted. The lumen is related to the candela as:

1 lm = 1 cd ⋅ sr [steradians; 球面度]

球面度是弧度的三维版。Radians and steradians are dimensionless [无量纲的], so "rad" and "sr" are included only for clarity. In mathematics, a unit circle is a circle of unit radius - that is, a radius of 1. 单位圆的半径没有单位。

立体角(solid angle)像是平面角的三维版。Solid angles can also be measured in squares of angular measures such as degrees, minutes, and seconds. 立体角也可以用角度测量的平方来测量，例如度、分和秒。

Candela就是拉丁语里的candle. Nit comes from the Latin nitere, which means “shine.” Candela /kænˈdɛlə/ or /kænˈdiːlə/ ɛ是美式发音符号，e是英式，读音是一样的。

Lux (lx) is a brightness measurement unit that represents the quantity of light that is received by a surface and expressed in lumen per square meter:

1 lux = 1 lm / m²

Luminance is a photometric [光度] measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through, is emitted from, or is reflected from a particular area, and falls within a given solid angle. Brightness is the term for the subjective [主观] impression of the objective [客观] luminance measurement standard. 但是：How to Set the Brightness and Contrast on Dell Monitors; Luminance is used in the video industry to characterize the brightness of displays.

A typical computer display emits between 50 and 300 cd/m² (nit). The sun has a luminance of about 1.6×10⁹ cd/m² at noon.

坎德拉（物理单位）- 百度百科 坎德拉是一光源在给定方向上的发光强度，该光源发出频率为540×10¹²赫兹的单色辐射，且在此方向上的辐射强度为1/683瓦特/球面度。

波长 = 波速 × 频率；光速 = 3×10⁸ m = 3×10¹⁷ nm; 该辐射的波长是555.6 nm，是绿光。

A full sphere has a solid angle of 4π steradian, so a light source that uniformly radiates one candela in all directions has a total luminous flux [通量] of:

1 cd × 4π sr = 4π cd⋅sr ≈ 12.57 lumens

1/683瓦特/球面度 × 4π sr  ≈ 0.018389 瓦特

但是蜡烛不是只发出频率为540×10¹²赫兹的光；我没见过发绿光的蜡烛。蜡烛和白炽chì灯发热都很厉害。

某LED球泡，耗电9W，亮度与63W白炽灯相当，标称光通量为850 lm，和850 / 12.57 = 68支蜡烛一起点差不多一样亮。说“差不多”是因为：

• 规格说明里还有光束角
• ...a Lambertian emitter that emits the same luminous Intensity in all directions. (Of course, this is almost never true in practice.)