In order to study the processes that (strongly) enhance the star formation rate in some galaxies we have selected 62 galaxies from the IRAS point source catalogue with far-infrared to blue luminosity ratios (L(FIR)/L(B)0) of three or larger. These galaxies form a subset of a far-infrared complete sample of 254 sources in two fields covering in total 1500 square degrees of the southern sky (10h less-than-or-equal-to alpha less-than-or-equal-to 14h and -40-degrees less-than-or-equal-to delta less-than-or-equal-to -20-degrees; 20h less-than-or-equal-to alpha less-than-or-equal-to 22h and -40-degrees less-than-or-equal-to delta less-than-or-equal-to -20-degrees). The sources in the far-infrared complete sample all have flux densities larger than 1 and 2.5 Jy at 60 and 100-mu-m, respectively, to ensure completeness. Furthermore, in order to select mainly galaxies, for all sources the flux densities at 60 and 100-mu-m are larger than at 12 and 25-mu-m. We initially used B-band magnitudes estimated from ESO/SRC and POSS sky survey plates to estimate L(FIR)/L(B) ratios for all galaxies in the far-infrared complete sample, and we selected only those galaxies for the optical observations that have L(FIR)/L(B)0 greater-than-or-equal-to 4.3, thus obtaining a sample of 62 galaxies. Comparison of the estimated with the observed, corrected CCD B-band magnitudes later revealed that the actual L(FIR)/L(B)0 selection criterium is less sharp and corresponds to L(FIR)/L(B)0 greater-than-or-similar-to 3. For these 62 sources we also obtained far-infrared co-added flux-densities, so that 100%, 95%, and 70% of the sources are detected at 60 and 100, 25 and 12-mu-m, respectively. Due to the L(FIR)/L(B) selection criterion and the correlation between L(FIR)/L(B) and the S100/S60 flux density ratio the subsample of 62 galaxies consists only of warm sources with log (S100/S60) less-than-or-equal-to 0.4. The optical observations comprise long-slit CCD spectra in the 4800-7300 angstrom wavelength range, broad-band B, V and R, and narrow-band H-alpha CCD images. In this paper we present the optical observations and show spectra, and broad-band, colour and continuum-subtracted H-alpha CCD images for more than 50 galaxies. The spectra are mostly HII-types although some 25% are AGN-types. The broad-band images show, among other things, that our sample predominantly consists of barred spirals and distorded interacting system, while the H-alpha images show in most cases small compact sources. From the spectra we derived redshifts, line-ratios, equivalent widths, and extinction determinations from the H-beta/H-alpha line ratios. We also derived integrated magnitudes, colours, H-alpha fluxes, diameters and luminosities of the galaxies from the CCD images. We determined average luminosity profiles of all early-type galaxies and all possible merger candidates in our sample, and we found that all these galaxies have exponential disks except two objects that follow a de Vaucouleurs r1/4 law: a merger candidate, and an elliptical galaxy in a close interacting pair (the latter is not an IRAS galaxy, however).
