Brightness of light sources I would like to know what determines the brightness of light. I'm confused, After hours of reading, I got these definitions mixed up I need to link them together: 


*

*Light intensity 

*Brightness of light 

*Number of photons 

*Energy of each photon

*Wavelength of each photon

*Power of light source


Then I knew that there are different types of cone cells that respond to different sensitivity, does that mean less sensitive cone cells need more photons to respond and a number of photons is lost? And how is that related to light brightness?
 A: Intensity is an objectively measurable attribute of light.  It is the rate at which energy is delivered to a surface.  Intensity is energy delivered per unit time per unit area.  The intensity of light is a measurement of photon irradiance, which is the number of photons delivered per square meter per second.  You can measure intensity with a photoelement, such as a solar cell or a photomultiplier, which converts light to electric current.  As the electric current varies strictly with the intensity of the light, an objective measurement is possible.  The more energetic the photon (the shorter its wavelength), the fewer will be the number of photons required for a given intensity.  See this link: http://www.pveducation.org/pvcdrom/properties-of-sunlight/photon-flux.  
Brightness is a "subjective" quality of light.  It depends on the perception of whoever is viewing the light.  It can't be objectively measured, but it can be scaled, so that the same viewer (or viewers with similar perceptions) can agree that certain light is more or less bright.  For instance a less bright surface may be deemed 50% of the brightness of a brighter surface.  In astronomy, for example, stars may be graded according to their apparent magnitude, which is their brightness in comparison to a very bright benchmark star.  Brightness may also be called luminous flux.  Here is a list of units in which various luminous standards of brightness are scaled: https://en.wikipedia.org/wiki/Luminous_flux
Photon is a term used to describe the particle attribute of light.  A photon may be considered the smallest packet of energy into which light can be separated.  
You could deliver greater intensity by emitting light of a shorter wavelength, or by increasing the surface area emitting the light (greater surface area means more electrons emitting more photons).
The energy of each photon is measured in joules, and depends on the wavelength of the light according to this formula: Q = h*c / lambda, where Q is energy in joules, h is Planck's constant, c is the speed of light in a vacuum, and lambda is the wavelength of the light in meters.  Photons are considered to be "massless particles", but since a photon has energy, it must have mass.  This conundrum is solved by saying that photons have relativistic mass when they are traveling, but possess zero rest mass because they are never at rest.  Here is a better explanation: http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html.  
The wavelength of a photon determines its energy, and its energy determines its wavelength.    The shorter the wavelength, the shorter the cycle of the wave, the greater its frequency, and the more periods of the wavelength can be crammed into a unit time.  Short wavelengths are more energetic than long wavelengths.  As photons of light possess both particle and wave natures, a photon when traveling has velocity (c), momentum (relativistic mass * c), and wavelength, but when a photon is emitted or absorbed (at the beginning and the end of its journey), it is considered a particle.  
The power of a light source can be measured in watts.  A watt is the rate of energy of one joule per second.
