2.5 Filters

Information on the SW and LW filters is given in Tables 2.3 and 2.4. The reference wavelengths were chosen to clearly identify the different filters. Generally they are easy to remember wavelengths, and in most cases they are close to the isophotal wavelength for either a $\lambda F_{\lambda}$ = constant spectrum or the energy distribution of Sirius (Moneti et al. 1997, [42]). These definitions have also been applied to the ISOPHOT filter system. The transmission curves are given in Appendix A. The spectral transmission curve for each filter is the filter transmission convolved with the detector quantum efficiency. The method for colour correction of observed flux densities is explained in Appendix A. The flux calibration of the filter measurements is described in Section 4.1.1.

**Table 2.3:** SW filter characteristics. $\lambda_{ref}$ is the reference wavelength (Moneti et al. 1997, [42]), while $\lambda_c$ is the central wavelength of the transmission curve. The range of wavelengths between the half-maximum transmission points, the approximate spectral resolution and the main scientific purpose are also indicated.
Ident.	$\lambda_{ref}$	$\lambda_c$	$\Delta \lambda$	$\lambda$ / $\delta \lambda$	Scientific purpose
	[ $\mu m$ ]	[ $\mu m$ ]	[ $\mu m$ ]
SW1	3.6	3.57	3.05-4.10	3	Cosmological filter, close to L'
					Common to ISOPHOT
SW2	3.3	3.30	3.2-3.4	17	PAH (3.28 $\mu m$ )
SW3	4.5	4.50	4.0-5.0	5	General purpose, common to LW
SW4	2.8	2.77	2.50-3.05	5.5	General purpose
SW5	4.0	4.25	3.0-5.5	2	High sensitivity
SW6	3.7	3.72	3.45-4.00	7	Optimum L, comparison for SW7
SW7	3.0	3.05	2.9-3.2	10	Ice band
SW8	4.05	4.06	4.00-4.15	33	Br $\alpha$
SW9	3.9	3.88	3.76-4.00	20	Comparison for SW8
SW10	4.6	4.70	4.53-4.88	13	CO fundamental
SW11	4.26	4.26	4.16-4.37	20	CO

**Table 2.4:** LW filter characteristics. $\lambda_{ref}$ is the reference wavelength (Moneti et al. 1997, [42]), while $\lambda_c$ is the central wavelength of the transmission curve. The range of wavelengths between the half-maximum transmission points, the approximate spectral resolution and the main scientific purpose are also indicated.
Ident.	$\lambda_{ref}$	$\lambda_c$	$\Delta \lambda$	$\lambda$ / $\delta \lambda$	Scientific purpose
	[ $\mu m$ ]	[ $\mu m$ ]	[ $\mu m$ ]
LW1	4.5	4.50	4.0-5.0	5	General purpose, common to SW
LW2	6.7	6.75	5.0-8.5	2	General purpose
LW3	14.3	15.0	12.0-18.0	3	High sensitivity
LW4	6.0	6.0	5.5-6.5	6	UIR feature
LW5	6.8	6.75	6.5-7.0	14	General purpose, comparison for LW4
LW6	7.7	7.75	7.0-8.5	5	UIR feature, comparison for LW7
LW7	9.6	9.62	8.5-10.7	4	Silicate band
LW8	11.3	11.4	10.7-12.0	9	UIR feature, comparison for LW7
LW9	14.9	15.0	14.0-16.0	8	General purpose, CO
LW10	12.0	10.5	8.0-15.0	2	IRAS Band 1, common to ISOPHOT