The solar radiation which reaches the Earth’s surface after going through the atmosphere is measured and analysed by man in diverse forms, given its importance as an energy source for our planet.  The physical processes which have been produced in the interaction between it and the atmosphere have been modelled on spectral processing to understand it, as its interaction depends on wavelengths. From the spectral modelling, broadband integrated models have been constructed which explain the transformation changes to the group of all solar spectral energy. At the same time broadband has been fully measured and observed, and the variability of its global and diffuse components has enabled the application of the mentioned models to learn through radiometric measurement about lower and higher aerosol levels, visibility conditions and cloudiness, among others of atmospheric conditions.
However, in general, certain spectral solar bands are important for man and living things, especially in some of the ultraviolet zones and the visible bands, or Chappuis’s, which has focussed the attention of researchers and has been fully measured and analysed. In the stages of radiometric measurement it is common to measure broadband irradiance, or instantaneous irradiation, and very often in the partial bands irradiance. Moreover, spectral irradiance is measured even though it is not as widespread as global or partial band irradiance.
Spectral measurement or including those that we could denominate monochromatic radiation is more precise and provides more information about the processes that the radiation is undergoing then its interaction with every atmospheric component is different to the wavelength function. However, when broadband irradiance goes through the atmosphere, the modelling is analysed in-depth, and for this there are coefficients, factors, and indexes which enable one to infer from its measurements an enormous amount of information about the atmosphere and its components. 
Thus, the Ångström coefficient ? was established to express the atmospheric turbidity level, and even though it is defined in relation to wavelength is a parameter of the entire band and then broadband measurement enable to determine this coefficient and find out the atmospheric turbidity in an instant. Also, the Linke Factor of TL was established for broadband irradiance with the finality of identifying atmospheric turbidity. Other established indexes for broadband radiation, such as, the clearness index or zenith angle independient clearness index  of the enables one to draw the borders between what is understood to be clear and cloudy sky.  These are among others, some of the applications obtained by means of the behaviour broadband modelization.
This investigation has focussed attention on broadband irradiance behaviour and has as an objective to apply the same methodology and establish the factors and adequate indexes to proceed with them and their parameterizations to the analysis and the irradiance modelling of  four spectral bands from the ranges 280-315 nm (UVB), 315-400 nm (UVA), 390-385 nm (UV) y 400-700 nm (PAR) which are of great interest, as each of them separately and independent from the rest of the spectrum, for living things and its vital functions.  
Previously in this investigation to the study of the mentioned bands and to establish the newly mentioned parameters and its application, which has been done firstly is the determination of ? and TL from broadband irradiance measurement in Valencia for an entire year in conditions of clear sky, to compare and study the behaviour of both parameters in the observation of the same yearly cycle. To obtain the first one have beeen used global and diffuse horizontal measurements, and to determine the second one direct normal measurements. In the selecting clear sky instances, an established criterion from the literature has been used for available broadband measurements. 
 The data base that was used in this investigation contains global and diffuse irradiance measurements and global irradiance measurements of the bands which are focussed on in this work, all of them on the horizontal surface and normal direct of the entire band through various years differents according to the wavelength analysed. Punctually, measurements were done on two month period to include diffuse horizontal irradiance of the four bands.
To transfer the methodology applied to broadband to the analysis of bands, and in particular a tool which provides the TL Factor have been defined the denominated Band Factor TB, as the ratio between vertical optical depth for this band and that corresponding to Atmospheric Clean and Dry (CDA), and in that instant, in a similar way that the Linke factor is established for broadband.  
To determine the mentioned factors which have been previously defined it has been necessary the parameterisations of the vertical optical depth of CDA in Valencia for each of the four bands. This enables the determination of TB from measurements of normal direct band irradiance. 
In the mentioned data base from the years 2000-2004 for the UVB band and from 1996-2004 for the UV band there were measurements available for the mentioned bands for global irradiance but not for direct irradiance. To make the first determination of TB taking into account the lack of direct irradiation, a method has been elaborated with the atmospheric conditions of Valencia, enabling the inference of the direct irradiance value from the measurement of global irradiance. This has required a previous parameterisation between the sets of these two components of the irradiance generated through a spectral model, in which predictions were first integrated for a band range and then afterwards for the other.
Once, elaborated this new data base has been able to determine for Valencia for the mentioned periods and in diverse circumstances the factors of the two mentioned bands. Its development and variability has been observed and selected the corresponding instances of a clear sky, to determine the maximum TB which is produced in each instance.     
With the purpose of having direct normal irradiation measurements to calculate more precisely the band factors and without the necessity of using the mentioned model, the mentioned intensive measurements campaign during the months of June and July in 2006 of global and diffuse horizontal irradiance of the four mentioned bands has been carried out, interposing a shadow disk in the instruments destined for the global measurement.
The band factors calculated from this mentioned measurements campaign have been correlated with the Ångström turbidity coefficient ? and with ozone content. Given that coefficient ? is an indicator of atmospheric turbidity and the regressions obtained against the differents band factors are acceptable, we can affirm that the band factor is a coefficient which also can indicate the mentioned turbidity. Thus, if we only have irradiance band data the calculation of the mentioned factor provides us information about atmospheric turbidity. In relation to ozone content, an observation of a different influence is seen of the absorption of solar radiation according to the UVB, UV, and PAR bands by its order of importance, better correlation of TB with ozone is produced for the UVB band, meaning that this is where the ozone has it greatest absorption and nearly the UV band.   
The basic clearness index of the band is defined, which we denominate as Kt’, and expresses the ratio of global experimental irradiance and the global standard for a clear instant in the mentioned band as was done by Pérez for the entire band. This index has been determined from our data base for the UVB and UV bands. The regression of these to those of the Pérez index is acceptable for the Kt’UV index and weak for the Kt’UVB index. Limited values have been found which separate clear and cloudy days for each of the bands, with the UV band showing best results in relation to the selection done with the Pérez broadband index.   
Another index has been established which we denominate the normalized clearness index of band which is named Kt’’ and expresses the ratio for normal direct experimental irradiance and normal minimum direct irradiance of a clear instant. To calculate this it has been necessary the parameterisation of the maximum band factor in relation to the relative optical air mass. This index was determined using our data base for the UVB and UV bands. The regression of these to those of the Pérez index, as it is dealing with distinct indexes, is only weak for the Kt’’UVB index and acceptable for the Kt’’UV index. From these, the limited values established for the UV band provide better results in choosing clerness and coinciding with 90% of the selection using Pérez index. 
Lastly, the utility of the normalized clearness index for the experimental data of the measurement campaign and the limited values of them that have been adopted to differentiate between cloudy and clear periods have been verified, and even though there are few instants of measurement during this campaign of what is known about direct observation its characteristics, discriminate well with a coincidence of 96.9% for the UVB and 98.9% for the UV compared to the selection made with broadband measurement  applying Richard Pérez index.
For the  analysis done on the new indexes Kt’ and Kt’’ we can conclude that this could be of great usefulness for the substitution of Pérez index as an indication of atmospheric clarity in a determined instant when one does not have broadband irradiance data.