3. Irradiance - Measured in watts per square meter, irradiance is the measurement of radiant flux on a known surface area.

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In photometry,  luminous intensity is a measure of the radiant power emitted by an object in a particular direction and is dependent on the wavelength of light being emitted.

When it comes to lighting, you need to consider watts (W) (energy used) versus lumens (lm) (brightness). Or electricity consumption versus light output. Lumens are weighted for human perception where as watts are not.

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Sciencing wrote a step by step article/experiment on How to Calculate Light Intensity with the intensity of light around a bulb that radiates light equally in all directions. The conclusion detailed that "the light intensity at your point on the sphere is equal to the number of watts that the bulb radiates divided by the surface area of the sphere." The full calculations can be found here.

4. Luminance - Measured in candela per square meter or nit, luminance is the total light emitted or reflected from a surface in a given direction. It indicates how bright we perceive the result of the interaction of the incident light and the surface.

"The basic function of a spectrometer is to take in light, break it into its spectral components, digitize the signal as a function of wavelength, and read it out and display it through a computer.”

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In an article written by ATA Scientific Instruments, What is Spectrometry and What is it Used For, they detail modern ways we use spectroscopy:

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Photometry is a subset of radiometry that only applies to the visible portion of the electromagnetic spectrum. While radiometry focuses on measuring radiant energy in terms of absolute power, photometry takes into account the response of the human eye and focuses on measuring light in terms of perceived brightness.

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It's important to understand the different terms used to characterize light. From the measurement of light in the electromagnetic spectrum, to understanding perceived brightness to the human eye, light intensity and the tools used to measure light, this guide covers it all.

2. Luminous intensity - Measured in candela, luminous intensity is the amount of light emitted by a source in a particular direction.

Think of it like this – IL-Luminance, IL, I = Incident Light. Illuminance is measuring the incident light. Luminance is what’s leaving the surface – L = leaving. Illuminance measures incident, luminance measures what’s leaving.

"An integrating sphere collects electromagnetic radiation from a source completely external to the optical device, usually for flux measurement or optical attenuation."

Overall, radiometry is the science of measuring electromagnetic radiation. In regards to optics, it refers to the detection and measurement of light waves in the optical portion the electromagnetic spectrum (infrared, visible, and ultraviolet). Radiometry also includes characterizing the distribution of the radiation's absolute power.

1. Luminous flux - Measured in lumens, luminous flux is the measurement of total perceived power emitted in all directions by a light source.

There are many terms and technologies used when it comes to the power of light and light measurement. It is key to understand how all of these unique aspects come together.

Spectroradiometry. In KonicaMinolta.us:  Radiometry, Spectroradiometry and Photometry Retrieved from: https://sensing.konicaminolta.us/learning-center/light-measurement/radiometry-spectroradiometry-photometry/

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4. Radiance - Measured in watts per square meter Steradian, radiance is the measure of radiant intensity emitted from a unit area of a source.

Calculating the intensity of light depends on the light source and the direction in which it radiates light. The amount of light falling on a surface is known as illuminance and is measured in lux.

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3. Illuminance - Measured in lumens per unit area, illuminance refers to the amount of light incident on a surface. Illuminance can also be referred to in foot-candle.

Understanding the measurement of light helps us, as a lighting solutions provider, meet the brightness and uniformity requirements of your specific applications.

Spectroradiometry is the "measurement of light energy at individual wavelengths within the electromagnetic spectrum. It can be measured over the entire spectrum or within a specific band of wavelengths."

As noted above, flux is the total light output. With watts referring to absolute power and lumens being weighted for human perception.

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What matters the most in terms of measuring light intensity is the actual number of lumens falling on a particular surface.

The diagram below shows how spectrometry is used to analyze a sample.  The sample is shown in Step 2.  Spectrometry can also be used to analyze the wavelengths present in a given light source.  In this instance, there would be no sample between the source and diffraction grating.

Spectral Irradiance - the irradiance of a surface per unit frequency or wavelength. The SI units for spectral irradiance is Watt/cubic meter.

Spectrometry is known for the science and utilization of spectrometers for measurement and analysis. It is the study of interactions between light and matter, and the reactions and measurements of radiation intensity and wavelength.

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Flux (Luminous Flux) - Originating from the Latin word 'Fluxus,' meaning flow, flux is the amount of energy a light emits per second, measured in lumens (lm).

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The biomedical use of light consists of diagnostic and therapeutic applications. Read more on Spectroscopy in Biomedical Services.

Photometry is distinguished from radiometry in which each separate wavelength in the electromagnetic spectrum is detected and measured, including the ultraviolet and infrared." Photometry. In EDU.photonics.com/Photometry: The Answer to How Light is Perceived Retrieved from https://www.photonics.com/a25119/Photometry_The_Answer_to_How_Light_Is_Perceived

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Photometry is the "science of the measurement of light intensity, where 'light' refers to the total integrated range of radiation to which the eye is sensitive.

image credit:  By publiclaboratory  Spectrometry diagram [CC BY 2.0] (https://creativecommons.org/licenses/by/2.0/), from flickr

Spectral Radiance - the radiance of a surface per unit frequency or wavelength. The SI units for spectral radiance is Watt/square meter Steradian nanometer.

2. Radiant Intensity - Also measured in watts, radiant intensity is the amount of flux emitted through a known solid angle.

1. Radiant Flux/Power - Expressed in watts, radiant flux can be defined as the total optical power of a light source. It can also be defined as the rate of flow of radiant energy. You can think of it as the total amount of light emitted from a light bulb.

A photometer is an instrument that measures light intensity. It can be defined as an instrument that measures visible light.

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As with radiometry, applications of photometry are also diverse. It is used in a number of industries to test the intensity of light produced by displays, instrument panels, night-vision devices and more.