5.1.11Determination of Characteristic Parameters

5.1.11.1 General

The characteristic parameters are determined for quasi-parallel, almost normal radiation incidence.

5.1.11.2 Test setup

The samples shall be irradiated by a beam whose axis is at an angle not exceeding $10^{\circ}$ from the normal to the surface. The angle between the axis and any ray of the illuminating beam shall not exceed $5^{\circ}$.

The characteristic parameters are as follows:

a) Spectral transmittance $\tau (\lambda)$, the spectral external reflectance $\rho_{\mathrm{o}}(\lambda)$ and the spectral internal reflectance $\rho_{\mathrm{i}}(\lambda)$ in the wavelength range of $300\mathrm{nm}$ to $2500\mathrm{nm}$;

b) Light transmittance $\tau_{\mathrm{v}}$, the external light reflectance $\rho_{\mathrm{v,0}}$ and the internal light reflectance $\rho_{\mathrm{v,i}}$ for illuminant D65;

c) Solar direct transmittance $\tau_{\mathrm{e}}$ and the solar direct reflectance $\rho_{\mathrm{e}}$;

d) Total solar energy transmittance (solar heat gain co-efficient SHGC) $g$; and

e) UV-transmittance $\tau_{UV}$ ;

When calculating the characteristic parameters of multiple glazing, the spectral data of each glass component is to be used instead of integrating data of the complete unit.

5.1.11.3 Performance requirements of optical measurements

Optical measurements in transmission and reflection require special care to achieve accuracy in transmittance and reflectance of about $\pm0.01$ .

The wavelength calibration and the photometric linearity of commercial spectrophotometers shall be checked periodically using reference materials obtained from metrological laboratories.

The wavelength calibration shall be performed by measuring glass plates which feature relatively sharp absorption bands at specified wavelengths; the photometric linearity shall be checked using grey filters with a certified transmittance.

For reflectance measurements, reference materials having reflection behaviour (that is, reflectance level and ratio of diffuse and direct reflectance).

Thick samples (that is, laminated glass or insulating units) can modify the optical path of the instrument's beam as compared to the path in air and therefore the sample beam hits an area of the detector having a different responsivity.

A similar source of inaccuracy occurs in case of samples with significant wedge angles which deflect the transmitted (and reflected) beams. It is recommended to check the reproducibility by repeating the measurement after rotating the sample.

Additionally, in the case of reflectance measurements, glass sheets cause a lateral shear of the beam reflected by the second surface, causing reflectance losses (whose extent is particularly evident in the case of thick and/or wedged samples). This source of inaccuracy shall be taken into account particularly in the case of reflectance measurements through the uncoated side. In order to quantify and correct systematic errors, it is

recommended to use calibrated reflectance standards with a thickness similar to the unknown sample.

In the case of diffusing samples (or samples with a non-negligible diffusing component or wedged samples), transmittance and reflectance measurements shall be performed using integrating spheres whose openings are sufficiently large to collect the entire diffusely transmitted or reflected beam. The sphere diameter shall be adequate and the internal surface adequately coated with a highly diffusing reflectance material, so that the internal area can provide the necessary multiple reflections. Reference materials with characteristics similar to the unknown sample as specified above shall be used. If the transmittance or reflectance curve recorded by the spectrometer exhibits a high level of noise for some wavelengths, the values to be considered for those wavelengths should be obtained after a smoothing of the noise.

5.1.11.4 Light transmittance

Light transmittance is the measure of the amount of light passing through the glazing. The visible light lies between the wavelength 380 nm and 780 nm. The light transmission of the glazing depends on the substrate type and coating layer done on the base glass.

The light transmittance $\tau_{\mathrm{v}}$ of glazing shall be calculated using the following formula:

$D_{\lambda}$ = relative spectral distribution of illuminant D65;

$\tau (\lambda)$ = spectral transmittance of glazing;

$V(\lambda) =$ spectral luminous efficiency for photopic vision defining the standard observer for photometry; and

$\Delta \lambda =$ wavelength interval.

Table 10 indicates the values for $D_{\lambda}V(\lambda)\Delta \lambda$ for wavelength intervals of $10\mathrm{nm}$. The table has been drawn up in such a way that $\sum D_{\lambda}V(\lambda)\Delta \lambda = 1$.

In the case of multiple glazing, the spectral transmittance $\tau (\lambda)$ shall be obtained by calculation from the spectral characteristics of the individual components. Alternatively, measurements on non-diffusing multiple units may be performed using an integrating sphere. This may be achieved after reducing the interspaces under conditions that allow the collection of the whole transmitted beam.

The calculation of the spectral transmittance $\tau (\lambda)$ shall be performed using methods such as algebraic manipulation. Any algorithm that can be shown to yield consistently the correct solution is acceptable.

For the calculation of $\tau (\lambda)$ as well as for the calculation of spectral reflectance the following symbols for the spectral transmittance and spectral reflectance of the individual components are used:

$\tau_{1}(\lambda) =$ spectral transmittance of the outer (first) pane;

$\tau_{2}(\lambda) =$ spectral transmittance of the second pane;

$\tau_{\mathrm{n}}(\lambda) =$ the spectral transmittance of the $n^{\mathrm{th}}$ (inner) pane (for example, for triple glazing $n = 3$);

$\rho_{1}(\lambda) = \text{spectral reflectance of the outer (first) pane measured in the direction of incident radiation;}$

$\rho^{\prime}_{1}(\lambda) =$ spectral reflectance of the outer (first) pane measured in the opposite direction of incident radiation;

$\rho_{2}(\lambda) = \text{spectral reflectance of the second pane measured in the direction of incident radiation;}$

$\rho^{\prime}_{2}(\lambda) = \text{spectral reflectance of the second pane measured in the opposite direction of incident radiation;}$

$\rho_{\mathrm{n}}(\lambda) =$ spectral reflectance of the $n^{\mathrm{th}}$ (inner) pane measured in the direction of incident radiation;

$\rho'_{n}(\lambda) =$ spectral reflectance of the $n^{\mathrm{th}}$ (inner) pane measured in the opposite direction of incident radiation.

For the spectral transmittance $\tau (\lambda)$ as a function of the spectral characteristics of the individual components of the unit, the following formulae are obtained:

a) For double glazing, and

b) For triple glazing.

For multiple glazing with more than three components, relationships similar to above expressions are found to calculate $\tau(\lambda)$ of such glazing from the spectral characteristics of the individual components.

As an example for calculating $\tau (\lambda)$ according to the procedures of this Section, a glazing composed of five components may be treated as follows:

1. First consider the first three components as triple glazing and calculate the spectral characteristics of this combination;

2. Next, run the same procedure for the next two components as double glazing; and

3. Then calculate $\tau (\lambda)$ for the five component glazing, considering it as double glazing

PART 6 STRUCTURAL DESIGN — SECTION 8 GLASS AND GLAZING

consisting of the preceding triple and double glazing.

The light transmission requirement for the glazing should be selected in accordance with relevant Indian Standards.

NOTE — Normalized relative spectral distribution $D_{\lambda}$ of illuminant D65 multiplied by the spectral luminous efficiency $V(\lambda)$ and by the wavelength interval $\Delta\lambda$ . The values in this table are calculated according to the trapezoidal rule.