# What is Luminous intensity: Definition and 14 Discussions

In photometry, luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle, based on the luminosity function, a standardized model of the sensitivity of the human eye. The SI unit of luminous intensity is the candela (cd), an SI base unit.
Photometry deals with the measurement of visible light as perceived by human eyes. The human eye can only see light in the visible spectrum and has different sensitivities to light of different wavelengths within the spectrum. When adapted for bright conditions (photopic vision), the eye is most sensitive to greenish-yellow light at 555 nm. Light with the same radiant intensity at other wavelengths has a lower luminous intensity. The curve which measures the response of the human eye to light is a defined standard, known as the luminosity function. This curve, denoted V(λ) or

y
¯

(
λ
)

{\displaystyle \textstyle {\overline {y}}(\lambda )}
, is based on an average of widely differing experimental data from scientists using different measurement techniques. For instance, the measured responses of the eye to violet light varied by a factor of ten.

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1. ### Procedure to express radiation intensity of an LED in watts/cm^2

I am calculating Responsivity of a pn junction photodiode (a.k.a the target) by irradiating radiation from LED sources. For this purpose, i have two LEDs, one UV and another green LED. Note that LEDs are placed close to the target.UV LED : Manufacturer has given total radiant power to be 20mW...
2. ### I Lux-meter, candelas or lux, measuring nits in practice

My lux-meter can be used in two modes, candela and lux. How do I use it to accurately measure a display's luminance in nits? The screen isn't Lambertian, I have to assume this as an unknown. I'm interested in the luminance as seen by the eye looking at the screen perpendicular to it. Now, first...
3. ### I A ray crossing 2 media of different indices and energy conservation

The ##I_i## are the intensity of the rays, in other words energy per surface units per radians by seconds. The d##\Omega## are the solid angles The equation p75 isis what I don't understand. I suppose that each side represent the energy going and out of the surface dS but I don't understand...
4. ### Mirror box with 1 candela of luminous intensity light source

considering that a simple mirror may reflect 99.9% of the visible light
5. ### A Color of Deep Space: Distribution of Light Wavelengths

I'm looking for the distribution of all wavelengths (or frequencies) of light that a stationary observer would receive at his location (at ##r = 0## and time ##t_0##), from all light sources emitting a single wavelength ##\lambda_{\text{e}}## (or angular frequency ##\omega_{\text{e}}##). The...
6. ### Luminous intensity: from Volt to Candela/m2

Homework Statement I use a probe for converting optical power to an electrical current. I need to convert Volt to candela/m2. The sensor, a photodiode, measure the brightness of the monitor leaning the probe on the glass (so there is no distance between the monitor and the sensor). The sensor...
7. ### UVA vs UVB: Why does UVA penetrate deeper into the skin?

Hello, My name is Chris and I am trying to understand why UVA radiation, with a longer wavelength, penetrates deeper into the skin than UVB which has a shorter wavelength. Since E=hv=hc/ λ and , then how does a lower energy and lower intensity UVA wave penetrate deeper into the skin than a...
8. ### UVA/UVB Light: Why does UVA penetrate deeper into the skin?

[Mentor's Note: Thread moved from New Member Introductions] Hello, My name is Chris and I am trying to understand why UVA radiation, with a longer wavelength, penetrates deeper into the skin than UVB which has a shorter wavelength. Since E=hv=hc/ λ and then how does a lower energy...
9. ### Stefan-Boltzmann law, luminosity, brightness and magnitude?

From what I understand, in the equation P=\sigma AT^4, P is the power output of the star which is the energy radiated per second in EM radiation of all frequencies, and I think luminosity is also defined as the energy radiated per second in EM radiation of all frequencies. Therefore luminosity...
10. ### Unit solid angle and finite angle

Homework Statement A point source emits visible light isotropically. Its luminous flux is 0.11 lumen. Find the flux whithin the cone that has half angle of 30 degree from the light source. Homework Equations luminous flux = luminous intensity * solid anlge The Attempt at a Solution I tried...
11. ### Should luminous intensity be a fundamental unit?

It seems to me that luminous intensity should really be put in terms of energy, not a special unit (which itself is based on some arbitrary specification of energy.) The other 5 units and Avogadro's number should be the only fundamental units.
12. ### Luminous intensity due to an isotropic point source of light

Does the luminous intensity due to an isotropic point source of light at a point on a surface depend on the angle it makes with the normal to the surface?
13. ### Luminous intensity in different directions

# The intensity of direct sunlight on a surface normal to the rays is Io. What is the intensity of direct sunlight on a surface whose normal makes an angle of 60 degrees with the rays of the sun? I think the answer is Io itself because for an isotropic point source of light the luminous...
14. ### SI unit for luminous intensity

This is the SI unit for luminous intensity. The definition relates to blackbody radiation emitted at a certain temperature for a certain material (so I guess it ISN'T blackbody!), platinum, I think. Except... I don't understand the necessity for the introduction of this unit. Isn't...