The relationship between the structure of the host glass and the properties of the doped ions is useful for the design of glasses for the different applications. Well defined and sharp emission lines of rare earth ions may serve as structural probes for the environment of the dopant. The ultraviolet transparency of insulating glasses depends on the glass formers for the silicate

borate and phosphate conventional glasses. So this work studies how it does in calcium aluminosilicate glasses when MgO or La

2

O

3

were added. It reports the absorption edge shifts and the changes of the optical band gaps were observed for Er

3+

doped CAS glasses by adding MgO and La

2

O

3

in them. Glasses with the batch composition (a) 9SiO

2

·26Al

2

O

3

·65CaO·1.0Er

2

O

3

·(0.3Yb)

2

O

3

(b) 9SiO

2

·26Al

2

O

3

·58CaO·7MgO·1.0Er

2

O

3

·0.3Yb

2

O

3

and (c) 9SiO

2

·26Al

2

O

3

·58CaO·7MgO·1.0Er

2

O

3

·0.3Yb

2

O

3

·5La

2

O

3

were melted at around 1500℃ and annealed at temperatures near glass transition temperature for 4 h. Then the glass was cut and polished for spectroscopic measurements. The (refractive) index

the density and the absorption spectra were measured. The ionic packing ratio and the (Judd-Ofelt) intensity parameters are calculated. The absorption edge gives a measure of energy gap. The fact that the fundamental optical absorption edge for samples shifts towards shorter wavelengths with adding of MgO and La

2

O

3

contents signifies the role of MgO and La

2

O

3

in CAS glasses. Ω

2

Ω

6

and

E

opt

increase with the adding of MgO and La

2

O

3

. The Urbach energy obtained are consistent with the trend of

E

opt

values.