Among traditional Indo-Persian style helmets used in Islamic and some Indian armies, one of the distinct helmet elements is a central spike at the apex of the helmet. Traditionally this spike is pyramidal, or spear shaped. Among the helmets of the Sikh Empire, the overwhelming design element seen at the helmet apex is not a spike, but a porte-aigrette plume holder designed to hold high a kalgi plume on the wearers head.
Herron Plumes "Kalghi" - the helmets of the Sikhs are ornamented with one or three plumes of black heron's feathers which are prized, one feather only being found in each wing. Their stems are bound with gold and silver wire. (Egerton, 1880) 
The tri-porte-aigrette configuration with a kalgi plume at the apex of a helmet and two other kalgi plumes is also found on helmets from other regions of South Asia, but it seems to have been an important and significant element in the design of helmets from Punjab.
The steel helmet is of a hemispherical shape with gold lines separating ribs which radiate from the damascened heron plume holder crowning the top. The forehead band is decorated with gold kuft work, and in front are two heron plumes with an ornamental bar between them, which can be lowered to protect the nose. (Cole, 1874) 
The kalgi plume was typically made from slender black heron feathers mounted on sticks, with gold gimp in circular sprays, and called batkha-kalgi. 
The high regard that Sikhs had for the kalgi plume may date back to the time of Guru Hargobind (1595 – 1644), the 6th Guru of the Sikhs. As a symbol of temporal authority as a Guru and as a King he wore the ornaments of royalty which may have included a plume and the Guru was referred to by Sikhs as ‘Sacha Padshah’ (The True King). Guru Gobind Singh (1666-1708) the 10th and final living Guru of the Sikhs is also often referred to by Sikhs as ‘Kalgiaa(n)vala’, meaning the one who wears the plume.
The importance placed on the kalgi plume seems to have become an integral part of the headgear of high ranking members of the Sikh Royal Court at Lahore as well as senior officers in the army of Maharaja Ranjit Singh.
Plumes have been made of herons' feathers in many countries, and here they are in great request, as no Sikh officer of any rank is ever seen without one in his turban ; and in those of the Surdars, or great men, the feathers, generally an uneven number, between ten and twenty, are fastened into a funnel-shaped stem, entirely covered with gold wire or thread, or sometimes richly ornamented with pearls, emeralds, and rubies. (Vigne, 1842) 
Their fondness for showy and gaudy colours is excessive, especially amongst the higher class; and usually consists of a yellow, orange, crimson, or other bright-coloured robe, with a turban to correspond: most of these articles being composed of silk stuffs. The majority carried a spear or sword, and shield; and a few wore a curious sort of plume, about a foot high, and top of which, from its arrangement, very much resembles the flower of the cockscomb, but the remaining parts being bare, gives it too much of the ragged appearance to look well. (Barr, 1844) 
1. Wilbraham Egerton, An Illustrated Handbook of Indian Arms (London: 1880), 69
2. H.H. Cole, Catalogue of Objects of Indian Art Exhibited in the South Kensington Museum (London, 1874), 148
3. G. N. Pant, Indian Arms and Armour, Volume III (New Delhi: Army Educational Stores, 1983), 46
4. G. T. Vigne, Travels in Kashmir, Ladak, Iskardo, Volume 1, (London: 1842), 306-307
5. William Barr, Journal of a March from Delhi to Peshawur (London: 1844), 55