###### Abstract
A review of the literature shows that only two such indexes have been investigated and proposed as possible candidates to meet this requirement: the _blue fraction_ (BF) and the _average photon energy_ (APE). BF has been introduced to assess the ratio of spectral irradiance integrated below 650 nm to the total irradiance [15], whereas APE is the mean energy of all the photons that impinge on a given surface [16]. Despite some early promising results presented by [PERSON] et al. [17] and later limited results reported by [PERSON] and [PERSON] [18], the possible one-to-one relationship between BF and spectral irradiance still remains highly unexplored. Indeed, this issue has been scarcely addressed thus far [8; 18], therefore justifying further investigation. In contrast, APE is an older and more popular index amongst the PV community. It has been widely used to provide a readily qualitative assessment on whether a specific spectrum is shifted to shorter or longer wavelengths relative to the standard spectrum. Thus, a more in-depth analysis of this parameter, including its physical significance, is provided hereafter. [PERSON] et al. [19] first analyzed the relationship between APE and the spectrum shape of global tilted irradiance (GTI). Their experimental campaign was conducted in Kusatsu City, Japan (latitude 34\({}^{\circ}\)58\({}^{\prime}\) N) over a period of three years. Spectral irradiance was monitored on a 15.3\({}^{\circ}\)-tilted surface facing the equator and led the authors to the conclusion that the relationship between APE and the GTI spectrum was bijective. Their methodology was based on an International Electrotechnical Commission (IEC) standard [20], which states by how much the output of a solar simulator can deviate from the AM1.5G reference spectrum. Their reasoning was basically that, if spectral distributions grouped in very narrow APE intervals have a very small standard deviation (SD) in all considered 50-nm bands, such distributions may be considered equal. A closer insight to that methodology is provided in what follows. Subsequent work assumed this one-to-one relationship between APE and GTI [21; 22; 23; 24; 25; 26; 27; 28; 29; 30], although further investigation at other sites was encouraged to confirm the possible universality of such a relationship. [PERSON] et al. [31] carried out a statistical analysis very similar to that of [PERSON] et al. [19], using measurements of spectral global horizontal irradiance (GHI) collected at two sites with different climates: Ispra, Italy (latitude 45 deg49' N, longitude 8 deg36' E) and Golden, CO, USA (latitude 39 deg45' N, longitude 105 deg13' W). Measurements were recorded for 16 months at the latter location and over two years at the former, using dissimilar instruments and experimental protocols at each site. [PERSON] and coworkers concluded that the relationship between APE and the GHI spectrum shape was bijective. However, these authors also raised the need for further collection and analysis of spectral irradiance data at other sites because firm conclusions could not be drawn from measurements at only two sites. In contrast, the relationship between the spectral distribution of GTI and APE was found _not_ bijective by [PERSON] et al. [12]. They noted that spectral distributions with the same value of APE might yield different values of MM for the same PV material. Their conclusion was based on the analysis of spectral GTI data collected on a 35 deg-tilt equator-facing surface over a 3.5-year experimental campaign conducted in Freiburg im Breisgau, Germany (latitude 47 deg59' N). A later study presented by [PERSON] et al. [32] applied the same methodology as proposed by [PERSON] et al. [19] to two spectral datasets collected on a 30 deg-tilt equator-facing surface during a two-year experimental campaign conducted in two Spanish cities with nearly the same Mediterranean-Continental climate: Jaen (latitude 37 deg49' N) and Madrid (latitude 40 deg24' N).