I think what you are referring you is 'intensity',
@benthamitemetric
To my understanding, and please correct me if wrong, but intensity in physical terms is the density of the radiating energy (the energy flux). In common language we call that "brightness" when talking about light.
More brightness does not affect the wavelength, ie. the colour of the light because more brightness/intensity simply means the amount of photons flowing through a given area is increasing, and we perceive that increase as "brighter". But that does not affect wavelength.
Just as more photons hit our eyes, a sensor of a camera will also register more photons as being brighter. But again, it will not affect the wavelength and the colour of the light.
For example, if the intensity is very low, there won't be enough photons for human eyes to register, even if individual photon wavelengths are very short and thus high in energy. It's similar to how extremely hot gas molecules won't produce any sensation of heat if the density of the gas is extremely low.
'Emissivity' however is merely the ratio of how well a black body is able to release thermal energy in accordance to the Stefan-Boltzmann law. 0 means no energy, and 1 means maximum possible amount for the given temperature.
What we perceive as white can come from two things:
• From a wavelength perspective it's the "sweet spot" where the red, green and blue wavelengths are in balance.
• Sensory overload, or overexposure. In such case the wavelengths does not need to be in balance as long as all wavelengths comes in intensity greater than the eye or exposure settings in a camera can handle.
Thus, estimating temperature by colour from photos can be tricky and misleading because:
• Cameras can have various exposure settings and limit in dynamic range (as shown in
@Mick West's experiment with the two cameras).
• Cameras have white balance settings that will offset what the human eye consider to be white, essentially distorting all colours in the photos.
If you don't understand these things it can be confusing when watching illustrations as the one below:
Like, the reference chart used by Mick West in his experiment shows 1300°C (1 573k) being white, but in this chart between 5000-6000k is illustrated as white.
The difference as I understand it is that - intensity, the amount of photons, will overload the naked eye around 1300°C, thus making it appear white to us. But from a strict wavelength perspective, only somewhere between 5000-6000k (which happens to be the frequencies involved in normal daylight) will appear white to us because the red, green and blue light receptor cells in our eyes will be equally stimulated.
In other words, if you overexpose a photo of something glowing at much lower temperatures (letting more photons hit the sensor, increasing brightness), it can still appear white, and this needs to be considered when evaluating colour temperature from photos where no camera metadata is included.