## 1 Introduction  
Ghost Modalities (GM) as Ghost Imaging [1; 2; 3; 4; 5; 6; 7; 8], Temporal Ghost Imaging [9; 10; 11; 12], Ghost Spectroscopy [13; 14; 15] and Ghost Polarimetry [16] are photon correlation metrology techniques and they represent an interesting topic in quantum optics. Ghost Imaging (GI) was the first ghost modality, realized for the first time in 1995 with entangled photons generated by Spontaneous Parametric Down Conversion (SPDC) [6; 13], thus twin photons in the quantum mechanical sense [6]. This quantum imaging modality exploits photon correlations for the image construction or reconstruction, where one photon of an (entangled) pair interacts with the object to be imaged and the experimentally determined correlation with the second photon yields the image, therefore it is called correlated two-photon imaging. Thus, a ghost image is obtained by correlating the light beam reflected or transmitted by an object with a highly correlated reference beam, which itself, however, had never interacted with the object [17; 18]. After the experimental demonstration of classical GI in 2004 [7; 19; 20] it became clear that GI cannot be understood as an exclusive quantum effect based on the two-photon interference capability of entangled light. In contrast to the experimentally rather sophisticated entanglement source concept, classical GI was originally based on so-called pseudo-thermal light, generated by a spatially randomly modulated coherent laser beam (the so-called Arecchi or Martienssen lamp: a combination of a rotating diffuser and laser light) [21; 22; 23]. This light beam is subsequently divided by a beam splitter into two beams which are mutually highly correlated and exhibit high spatial intensity-intensity correlations with two-photon interference capability, a fact well known since the fundamental Hanbury-Brown & Twiss experiment [24]. The capability of classical thermal light to mimic entanglement properties led to numerous experimental and theoretical investigations [4; 25; 26; 27; 28; 29]. Later, GI was realized with a so-called true thermal light source, a filtered neon discharge lamp [30], or even with the sun [31] as light source. In 2017, an ultra-miniaturized semiconductor-based broad-area superluminescent diode (BA-SLD) was proposed and introduced to the field as a new compact light source for classical ghost imaging having intrinsically all requirements of a GI light source [32]. Very recently, the classical GI concept based on the exploitation of spatial correlations was transferred into the spectral wavelength domain [14], thus realizing for the first time a ghost spectroscopy (GS) experiment with classical thermal photons by exploiting spectral wavelength-wavelength correlations of spectrally broad-band amplified spontaneous emission (ASE) light [33] emitted by an ultra-compact, miniaturized, optoelectronic, semiconductor-based Quantum Dot Superluminescent Diode (SLD) [14]. In the spirit of the Hanbury-Brown & Twiss experiment [24], this light showed up to exhibit a spectral second-order correlation coefficient of two, thus spectral photon bunching, one of the key requirements of GS. For the demonstration of the applicability of this SLD light source for GS with classical light, a \"real-world\" absorption spectroscopy experiment had been conceived at liquid chloroform, resulting in the central second-order correlations coefficient \(g^{(2)}(\tau=0,\lambda_{ref})\) as a function of the reference wavelength \(\lambda_{ref}\), thus the ghost spectrum, which exhibited all the characteristic absorption fingerprints \(S(\lambda)\) of chloroform at 1214 nm [14]. This so-obtained ghost absorption spectrum has been consequently interpreted as the analogue to the ghost image, but there in the spatial domain. This first demonstration of GS with classical thermal light in analogy to GI closed a gap in the experimental photon correlation modalities.