Good question, tunalover. These terms are used often in solar applications.
Emissivity, or "emissive power" [E(b)], is the rate at which radiant energy is given off by a body per unit area per unit time (often in W/m^2). Emittance (epsilon), on the other hand. is a dimensionless ratio – the ratio E:E(b) – that is, the ratio of the emissivity of a body to the emissivity of a perfectly-emitting and absorbing (theoritical) black body. Thus, emittances are always less than 1.0. Now, to confuse things further, emittances of specific surfaces are often stated AT A CERTAIN, DEFINED TEMPERATURE (sometimes termed "monochromatic wavelength", lambda). This is epsilon (subscript lambda). For instance, fresh snow is said to have a long-wave (thermal band) emittance of 0.82, while epsilon(lambda) for red bricks is 0.92. And black cupric oxide on sheet aluminum has an epsilon(lambda) of only between 0.09-0.21. Of interest to solar engineers is another ratio – the ratio of monochromatic short-wave absorptance to long-wave emittance, for it is desirable to have collector surfaces that absorb a maximum amount of the short-wave solar energy, yet emit relatively little (ie – most of the energy absorbed will be available for use via conduction to the desired application). Incidentally, absorptance (alpha) is itself a ratio of the amount of energy incident on a body to the amount of energy actually absorbed.
Transmittance is a term used in relation to glazing, and it is the fraction of (usually solar) radiation that a particular glazing layer allows through. Transparent surfaces reflect a certain amount of incident radiation, they absorb a certain amount, and they transmit the rest through to the other side. The relative percentages of each phenomenon depend on such variables as the angle of incidence, the chemical make-up of the glazing layer (which itself dictates such values as the index of refraction and the extinction coefficient), and the incoming wavelength. As far as I know, "transmissivity" refers to the same thing, for it is used interchangeably with transmittance in the standard texts.
Hope that helps. Sustainable, Solar, Environmental, and Structural Engineering: Appropriate technologies for a planet in stress.
I noticed, in re-reading after posting that I had made a typo: the subscript (b) for emissivity should not have been included in the first sentence in the above explanation. E(b) is the emissivity of a perfect "black body", whereas E is the actual emissivity, in W/m^2, of the particular "grey" body in question - at the defined temperature-wavelength range in which it is radiating energy. [One could say that my misplaced subscript "b" should have been subject to my omissivity.] Sustainable, Solar, Environmental, and Structural Engineering: Appropriate technologies for a planet in stress.
From "Radiative Heat Transfer", by Michael F. Modest, pp. 76:
"The National Institute of Standards and Technology (NIST, formerly NBS) has recommended to reserve the ending "-ivity" for radiative properties of pure, perfectly smooth materials, and "-ance" for rough and contaminated surfaces. Most real surfaces fall into the latter category, discussed in the present chapter. However, few researchers in the field appear to follow this convention, rather employing endings according to their own personal preference."
This is fairly consistent with what I've been told (and taught) over the past 15 years or so. Theoretically, there is a difference between emissivity and emittance -- but not practically. They are both ratios of actual emissive power to blackbody emissive power. The differences have more to do with the surface condition. Transmissivity and transmittance are also interchangeable.
